If say an average of 50 gliders could be made available over the 6 week summer holiday, and 4 GSs completed per aircraft per week, you very quickly complete 1,200 GS - very close to 50% the annual target for the entire organisation. Replicate that at Easter - potentially another 2 weeks as well as half term holidays, you see how you could maximise the delivery of training in the most efficient way possible.
...and in the process put hundreds of cadets in the air at the exact same time as every other man and his dog is puttering round the sky in flying machines various. This might cause some hesitation from the hierarchy. However your plan will never happen because, as you rightly say, there aren't enough instructors available and it's hard to see how this will change.