Could "kelner" be "server" as well? We seem to use "waiter" and "server" interchangeably when referring to restaurant staff in English.
Can "Biedny student" be used to mean (as the English can) a student who is not doing very well in his studies, exams, and so on, or is "biedny" only used for financial poorness?
2 times 'no' :D It can't be used for someone who 'does poorly' in his studies, however in a proper context it doesn't have to be 'financially poor', it could potentially be like "Oh, poor you!". This however will often use a noun, like "biedak" (Ooo, biedaku...) or even "biedaczysko" :D
Doing poorly in his studies is perhaps covered by the expression "On jest słabym uczniem" which has just come up further along this theme. Or that just mean literally weak? .
It's for 99% the 'doing poorly' interpretation - if he was literally weak, then what does that even have to do with the fact he's a student, why mention it? Then he's a "weak boy" or a "weak man" (or most probably simply "He is weak"), but "literally weak student" would be a strange thing to say in Polish.
But well, I say '99%', because technically you could try to interpret it literally. Very strange, but of course not impossible.
You also have "kiepski uczeń". "kiepski" is like "bad-ish", but it's actually used a lot more often than "zły" itself.