[OSM-newbies] Changes over time (demolished and abandoned buildings, felled trees)

[Someoneelse]

How to map stuff that isn't there any more has been discussed in the 
past (unfortunately without any consensus being achieved as far as I recall)

It might help to have a read of this on the wiki:
http://wiki.openstreetmap.org/wiki/Proposed_features/4th_Dimension
(and the link from there to earlier archived ideas)

and the "talk" list discussion that accompanied it:
http://lists.openstreetmap.org/pipermail/talk/2009-May/037135.html

If it was me, I'd at least mark the outline of e.g. the school with no 
tags other than a note saying what it is.  I wouldn't try and tag it as 
a school, because it isn't any more.  I'd also add a source tag saying 
where you got the info that there had been something there and how 
accurate it is (for example in the case of the tree - an out of 
copyright map could be quite a few metres out, but if you can poke a 
spade in the ground and hit what remains of the stump it's as accurate 
as your GPS is).