The building outline is fine, many thanks for adding it.

I am not sure how it could be tagged as a classroom, although amenity=community_centre might work if it's a community garden.

https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dcommunity_centre

If that doesn't look suitable and you wanted to tag it as more than a building, you could ask here:
https://community.openstreetmap.org/