Translation:Please don't go to school yet.
The particle に indicates direction. It can be translated as 'to' in this instance.
I'm still learning, but I think ください is very contextual. In some cases, it means "please" while in other cases it can mean "please give to me" or "can I [please] have" but either way it expresses politeness. In this case, I think an exact translation might be something like "can I [please] have you not go to school yet" but a more proper English sentence would be "Please don't go to school yet". Without the "please", the translation would sound like more of a demand than a request in my opinion.
Basically, yes. くださる is technically the 尊敬語（そんけいご） form of the verb くれる. Sonkeigo is respectful language, used to "raise up"/show respect to the person you're talking to - as opposed to 謙譲語（けんじょうご）, kenjōgo, which is humble language, used to "lower yourself"/show deference or humility when talking about yourself or your group.
The verb くれる directly means "to give", and is basically used when the person receiving the thing is being respected (i.e. usually when talking about something they're giving to you, or doing to help you). It can be used to either mean "give", or "to [do something] for [me/my group]" (basically "to give me the favor of doing [X]"). It's literally defined as: https://jisho.org/word/%E4%B8%8B%E3%81%95%E3%82%8B
- to give; to confer; to bestow
- to kindly do for one; to oblige; to favor
So effectively, ～てくれる is used when describing an action
~ that someone is doing for you (though that's the dictionary form of the verb; you'd usually use the -masu form, くれます). Changing くれる toくださる makes it more honorific/respectful towards the other person. ください is just the imperative form of that verb, so it effectively means "please give me" or "please do [X] for me".
See here for more info about 敬語（けいご）, respectful language: https://en.wikipedia.org/wiki/Honorific_speech_in_Japanese
Well, no. Potential form is when something has the possibility of happening. Here are the rules:
For う-verbs (G1), change the う-sound to an え-sound and add る. For example:
行く -> 行け -> 行ける
For る-verbs (G2), remove る and add られる (Basically put られ before る, or just れ for casual situations). For example:
食べる -> たべられる
Exceptions (G3) are できる for する and こられる for くる.
All potential verbs are る-verbs (G2 verbs), and can not take a direct object.
ある does not have a potential form, but it does have a verb-equivalent of ありえる, which I will skip over here.
The potential form would have a meaning like that of English, where "I go to the store" becomes "I can go to the store."
So after that explanation, I hope you can see that nowhere is the potential form used in this sentence; the ない form is used, as with 行かない, but nowhere is there 行ける or 行けない (negative potential).
Everything is matter, so it is matter if まだ goes after 学校に.
But no. More practically, no, it does not matter: まだ can go after and it will still convey the same meaning. I am unsure if it can go after the て形, given that ～てください is itself a single (arguably combined) grammatical structure, and it would make no sense to break up the grammatical structure (I have never seen it anyway).