I am also 98 kg and I avoid the likes of the C150/2 and PA38 as much as possible !
Seriously, 2 persons my size in one of these aircraft with full fuel is over MTOW, especially if you add in a couple of hefty flight bags. I would (and do) avoid schools that have these on their fleet and it gives me ease of mind. (See recent insurance thread re instructors). If you fly at over MTOW you are guaranteed illegal, certainly uninsured, hostage to fickle fortune and a bad example to the student. The aircraft will probably lift off eventually but do you really want a C152 to perform any worse than usual ??
Even with my preference, the PA28 Warrior II, you need to be careful, especially with C of G, as it is easy for two big lads (etc) up front to be out of limits balance-wise unless some ballast weight is carried in the rear. Check it out, if you do not believe me - or, better still, give the task to your student for a learning point well made.
As a final comment, if the worst happens and you are caught out over gross or out of balance you can put folding money on the flying school washing its hands of the matter. You are on your own ...
Sorry to sound so doom-laden but it W&B IS important.
Caveat aviator.