We have similar problems with my school woodland. because it backs onto a housing estate kids tend to think it is theirs to use as they want. Luckily the pond hasn't been touched but they do damage the trees for no reason, They run or walk through all the hedges just generally doing damage.
The school is having more CCTV's put in and thinking about getting a security person on site 24/7.. But how sad it is that things have to come to this.
Bring back national Service and the Birch.