Kaylee McIntosh, 14, of Fyvie, Aberdeenshire, became stuck under the hull of a capsized boat during a trip to Loch Carnan in the Outer Hebrides.
An investigation found an error during a head-count led to Kaylee being trapped by the boat for 90 minutes before anyone noticed she was missing.
Seven other cadets and four adults from the 2nd Battalion of The Highlanders Army Cadet Force managed to swim free from the vessel when it flipped over in August 2007.
A Fatal Accident Inquiry found a catalogue of failures had contributed to her death. It was thought the cadet struggled to get to the surface because she wore the wrong size life-jacket.
At Inverness Sheriff Court yesterday, Major George McCallum pled guilty to a charge under the Health and Safety at Work Act. He will be sentenced at a later date.
McCallum, now retired, from Peterhead, admitted Kaylee's unsuitable life-jacket caused her to be over-buoyant and that she was not properly accounted for, delaying rescue attempts and resulting in her death.
He admitted failing to take reasonable care for the health and safety of Kaylee and other cadets who were taking part in the exercise.
He admitted inadequately planning and assessing the risks of the exercise between July 26 and August 3, 2007 and failing to safely carry out the exercise.
Kaylee's parents, Derek and Lesley, said: "We welcome that Major McCallum has taken the only honourable route available and pled guilty to the charge against him arising from the significant role he played in the wholly unnecessary death of our beloved daughter Kaylee.
"By pleading guilty, Major McCallum has at least spared us the ordeal of sitting through a further legal hearing analysing the dreadful detail of Kaylee's last moments and the unforgivable catalogue of errors that led to her death.
"However, Major McCallum is only one of a number of people who have to answer for their actions on the day Kaylee was taken from us.
"Our campaign does not stop here and we will continue to press the Crown Office to bring charges against several others implicated in the death of our daughter."
Kaylee had travelled from the group's base at Boddam, near Peterhead, for an activity weekend with 34 youngsters.
The boat she was in was carrying 12 passengers – two more than the recommended limit – along with a heavy machine-gun.
A Fatal Accident Inquiry heard the craft was not designed for use in wind above force five, but was taken out on the loch in gale force eight conditions.
Sheriff Alasdair MacFadyen, in his inquiry findings, said the teenager drowned because of numerous failures.
He said planning, preparation and execution of the exercise had completely ignored written guidance.
The Marine Accident Investigation Branch found Kaylee was wearing the wrong size of life jacket, with three times more buoyancy than a child's aid. It pinned her underneath the boat.
Brigadier Paul Harkness, The Commander of 51 Scottish Brigade, yesterday said: "The Army is extremely sorry about the events that led to the tragic death of Cadet Kaylee McIntosh and for the loss suffered by her family in particular.
"We apologise unreservedly for any failures by the Ministry of Defence which contributed to her death. We continue to do our utmost to ensure that those involved have been kept informed of significant developments.
"The Ministry of Defence will continue to offer support to the McIntosh family for as long as they require it. The Ministry of Defence has already learned lessons from this tragic incident and taken steps to prevent a recurrence. When it becomes available we will review the Health and Safety Executive's report and take action to address any additional areas of concern.
"The Army remains committed to ensuring the many thousands of cadets who enjoy adventurous and other forms of training each year do so safely."
After yesterday's hearing, Elaine Taylor, head of the Crown Office Health and Safety Division, said: "The transportation of cadets by craft required adequate planning and assessment of the risks involved, and the accused failed in his duties to carry out both of these crucial elements to ensure that Kaylee and other cadets involved in the activity were safe.
"Those failings resulted in the entirely avoidable loss of the life of a young girl."