So has matey asked them if they're interested in using it and pitching in cost/effort wise to keep it basically maintained?
If they're not interested in using it, why the need for the padlock? And even if there were a justification for keeping the students out and deliberately and needlessly risking their lives in case of a fire, why has he not given OP a key - and made himself scarce when she's tried repeatedly to get hold of him?
It very much sounds to me like the determined garden grab was the primary goal and "but the students will probably trash it" was the weak excuse to try to justify it.
I don't know what these specific students are like, but I despise the lazy prejudiced assumption that all students are just good-for-nothing wasters and a general blight on the community.
Yes, most of them are young adults, living away from home for the first time - who might make some rash choices - but that's no justification whatsoever to demonise them as a drain on society - we've all been there at one point.
We wouldn't stand for a gross assumption that all old people (and I personally am much nearer that demographic than that of the students, fwiw) just go around complaining all the time, being generally a waste of space and getting in the way of the younger people who are still working for the good of society.