What does "in the back of" mean? Does it have the same meaning as "at the back of" which is opposite to "in/at the front of"? Can I say "John sits in the back of the classroom"? If so, I am wondering whether "in the back of" is British English and "at the back of" is American English. Am I right?
In the back of/at the back of
Yes, they are opposite to the phrases in the front of/at the front of. According to Ngram Review provided by TRomano, both are almost equally used in AE. However, in BE, the use of the latter is more usual.