"I am not a middle school student."
As a general rule Japanese individuals do not use subjects in their sentences so long as that subject has already been established. For them it's the equivalent of point at something and calling it out, which is considered rude.
As a foreign speaker they will not think less of you for always using a subject, or referring to yourself. However, they will be more impressed if you know the appropriate time to use a subject and when to drop it.
The rule is:
1) If the subject is understood or previously established do not use a subject.
2) If the subject is not understood, or is being changed use the subject once and precede from there.
I heard that the 'topic marker', は, is used when the subject (in this case, 'I', has already been introduced into the conversation. Otherwise, it would be が, which, as a 'subject marker', would be used to introduce a NEW idea or concept, would be used.
I have heard from 先生 types that one way of expressing '私は', is "... as for me, ..."
If you want to introduce a new topic entirely, out of left field, so to speak, such as your love for skiing, as a new thread of conversation, you might want to start with "私がスキー大好きです". (I love skiing).
I could be just wrong about this, I almost always see は following 私. I can see how it might be seen as egotistical to keep introducing one's self as the topic of conversation.
Anyone who understands this better than me is invited to correct me.
It's just that ではありません is the way of saying the opposite of です。Instead of saying 'I am', it is to say 'I am not'.
The counterpart of あります is ありません. That's "There is/There isn't". There are times in which あります means "I have"...but ask someone who speaks Japanese already when those times are. I'm just a student who has curiosity into this particular point of grammar.
中学生【ちゅう・がく・せい】… ～生（せい）a suffix that denotes student
年【とし、ねん】Usually ねん when denoting year as a suffix. It's not used in this sentence.
You can say 私は中学生いません but that most probably will be understood as simplified version of「私は中学生がいません」"I have no middle schoolers".
です and いる work differently, your confusion probably comes from Duo teaching ではありません which is a construction that negates です most of the time. So you can see ではありません、ではない、じゃありません、じゃない as the negative counterparts of です which usually involve a noun that connects to the whole structure.
いません in contrast, is the masu polite form of いる、a verb, this is very similar to "being" in English, but not quite. It's also used in other ways, the same happens with ある・ありません
So in short:
「中学生がいません」"Middle schoolers don't exist (here)" or "I don't have middle schoolers".
「中学生です」"I'm a middle schooler".
「中学生『ではありません』」"I'm not a middle schooler".
「中学生『じゃない』」"I'm not a middle schooler".
「私は、中学2年生じゃない -です」"as for me, (I'm) not a middle schooler in second year". The last です in this sentence is added to make the sentence sound more polite, is not like the other です。I know, confusing isn't it?
I gather that when questions are added to the system, the person adding them has to enter a whole load of allowable answers as well. Being only human, they can't think of every possible correct answer. But you can click the "REPORT" link if you think your answer should have been accepted, and it will be reviewed.
でせん is to my knowledge not a word. じゃない is used to express "is not" in casual situations. Sometimes じゃないです is used as well, particularly in situations where ではありません would be too formal, but じゃない would be too casual.
ではありません consists of the particles で and は (which is pronounced as "wa" because it is technically a particle), and the verb ありません, which is the negative form of あります which in return is the ます form or "polite form" of ある. To my understanding it literally means something like "does not exist in this way", but it's easier and recommended if you just remember it as a polite version of "is not" or the negative counterpart to です, because that is how it used and translated.
The negative of regular ある is a special case, which is simply ない. So ではない would be a casual version of ではありません, however in casual conversations the では is almost always contracted to じゃ which gives us じゃない.
Because of that, ではない and similarly じゃありません, while technically correct, are almost never used.
ではありません is the structure meaning "is not"
で marks a state of being, は is used for contrast in negative statements
ありません is the negative of the verb ある "to exist"
学生ではありません - The state of being a student does not exist - or more naturally, "I am not a student"
Replacing the state of being では with the subject particle が changes the meaning of the phrase
学生がありません "Students do not exist" or "There are no students" Though this has a strange nuance to it as usually particles like が and を are replaced with は in negative statements. It would be more natural to say 学生はありません for "There are no students"