They are both correct. Some native speakers will favour one, others the other. Personally, I would say "Back at school ...".

Hi dear Elena, Both combinations are used in both American and British English, though with some frequency differences. Back in school has 213 hits in Corpus of Contemporary American English, and "back at school has 111 hits. and the usages are almost the same in many cases. and the results are a bit different in the British National Corpus; the search has 43 hits for "back at school" and only 9 hits for "back in school"