Garden for pre nursery-capitalisation problem??

Yes it should be capitalised as the school is a newly organised one, and whatever expences to bring the area in to garden at the begining is capitalised.

Under which Head it should be capitalised?

A separate account-named Garden or merged with building a/c ????

mostly to Building or premisses

You can compare the building value and then if the Garden expenses is material you can show seperatly under land and buildings