After more than a decade leading Scotland's most prestigious talent school, there will be an encore for John Wallace:

he is going to pick up his trumpet in public again.

Before he was the effervescent principal of Scotland's leading music, theatre, and drama school, Glasgow's Royal Conservatoire of Scotland (RCS), formerly the Royal Scottish Academy of Music and Drama, John Wallace was known as one of the finest musicians Scotland had produced in modern times. The man from Methilhill, Fife, was the principal trumpet player of the London Sinfonietta, and principal trumpeter of the Philharmonia Orchestra. His own Wallace Collection was a highly successful brass ensemble, which toured and recorded extensively. And he was certainly the first former pupil of Buckhaven High School to perform at the nuptials of a prince and princess: he accompanied Dame Kiri Te Kanawa at the wedding of Charles and Diana at St Paul's Cathedral in 1981 with an audience in the hundreds of millions watching on television. Now, he says, his performance career is going to start again.

He is preparing to leave the institution that he has run, and that has also run his life, for the past 12 years. He leaves the RCS on August 31, after turning 65 in May this year. He will be succeeded by Jeffrey Sharkey, a director of one of America's foremost colleges of music: the Peabody Institute of the Johns Hopkins University in Baltimore, Maryland. But anyone expecting this non-stop dynamo of a man to potter off to his allotment (about which he is also, it has to said, excited) and wind down perhaps has not spent any time with Mr Wallace. He radiates enthusiasm, super-heated energy, and dynamic levels of sunny positivity. He is practising the trumpet again, working steadily up to the 24 hours a week of rehearsal that he feels he needs to perform at peak form again. He is receiving physiotherapy to help power up his diaphragm. He is also digging out sheet music that he wrote, for brass bands, in the early 1970s. He has some other plans up his sleeve, too, including bringing the Wallace Collection back to life for some new gigs.

We sit with tea in Wallace's corner office of the Conservatoire, which under his tenure has not only changed its name, but its curriculum and its presence in the city, with major new teaching and rehearsal buildings in the Speir's Lock area. "I am going to go off into other things," he says. "I am never going to actually retire, that is impossible. But the time had come for me to stand down - your energy gets slightly less, your appetites for doing the same things get slightly fewer. I had a lot of unfulfilled ambitions when I came into this job, and I still do, but in other directions. All your personal, artistic ambitions lie there, latent, and I hope I still have enough mental and physical powers to take them up again."

With his own compositions coming out of the cupboard, he says he is seeking the advice of the leading Scottish composer James MacMillan. "I have kept them quiet," he says, "I have put it in my bottom drawer. But I dug them out and I have said to Jimmy: can you have a look at this? Because, to be honest, I think it's c**p and I need some composition lessons again. So I will be composing again for brass bands and I will see if I can do that again." He is also shortly to perform with MacMillan again. He shows me a well-thumbed manual. "I have started to practise again. I have even got out the book of my mentor, The Art of Practice by Howard Snell. I have got it up to eight hours a week at the moment. I don't think I will ever get back to where I was in some ways, but in other ways I think I have improved. What has gone is the consistency - and that's why I have to practise." He sighs: "I am doing James MacMillan's Epiclesis in April at the Caird Hall and the Glasgow Royal Concert Hall." Epiclesis is a concerto for trumpet and orchestra, composed in 1993. "It's the hardest piece I have ever played," he says. He goes on: "Jimmy is to me is like Smetana, Janacek, Dvorak and Martinu [all Czech composers] all rolled into one. He has shown Scots that we do have a national style of composition and we have a national composer. I am just in tears a lot of the time listening to him: it's like when I heard the pipes when I was in exile [in London]."

Scotland and being Scottish are very important to Wallace. He is now very proud that the RCS is considered one of the top conservatoires, with a growing reputation abroad. He has a letter on his desk from Joseph Polisi, President of the Juilliard School of New York, thanking the RCS for its work and congratulating it on landing Mr Sharkey as its next principal. Wallace says: "In the first year of my job, he wouldn't talk to me: because we were invisible. But now we are visible. And that is good for Scotland. The name-change [in 2011] has worked internationally, but it is not quite embedded in Scotland, and we have a lot of work to do, which we will be doing over this year."

In 2001, he was living and working in London, but he sold up and left the different strands of his careers to accept the Glasgow post. Learning and discovery, as well as performing, are his passions. He had studied not only at Buckhaven High School but King's College, Cambridge, York University and London's Royal Academy of Music before arriving at the then-RSAMD, aged 52, as the first Scot to lead the institution for 100 years. What has driven him all these years? "I feel like I am playing for Scotland. I grew up when Scotland had a great football team. When I was 15 my father took me to Hampden to see Scotland playing England. I just live for those experiences - and trying to create them for this institution."

Wallace's first encounter with brass was the bugle of the rag man as he drove around his estate. His father, who worked in the Tullis Russell paper mill in Markinch, near Glenrothes, brought home a cornet from the mill band. His father, grandfather and uncles were all involved in the Fife brass band movement. Young John would try to emulate the rag man playing his bugle, but he was doing it wrong. His father "drilled" John but "every lesson ended in tears because my dad was so hard" so his mother sent him to the band. "I learned in a band of 40 kids, it was a huge group lesson and if you put a finger wrong, the leader would bash you over the finger. I learned a bit through corporal punishment."

At the age of eight he was playing with the Tullis Russell band and at nine he played at Edinburgh's Usher Hall. He earned a scholarship to Cambridge, sitting its entrance exam in a stationery cupboard at Buckhaven High.

He says: "I am horribly competitive. I think it has helped in this job. You have to have incredible stamina for a job like this: every day people are taking pot shots at you, even The Herald some days." He adds: "I was hard-wired, through my musical experiences in brass bands, to go for the best - every time. I am quite hard to contain at times, but people have appreciated that and wanted to work in that way as well."

Wallace is not leaving Glasgow. He has the "best of all worlds" he says - his daughter lives on Skye, his son in Salford, his father is still in Fife and his friends, home and wife Liz are in Glasgow. The RCS he is leaving is rather different from the one he joined. I ask him, finally, what he considers his most satisfying achievement at the RCS. He pauses, uncharacteristically, for some time. He says: "I am most proud of the up and at 'em attitude of the students when they leave here. When I first came here, a lot of the students were a bit cowed about performance. Now, the gallusness of the students - that is who we need to build the cultural economy of Scotland. That is what I am most proud of."