How do you say 学霸 in English?
學霸 describes an extremely academically proficient student. The best translation is probably straight-A or top student. Brainiac is very similar, but more generically implies a very academically clever person.

There are other words suggesting similar ideas, but some of them overlaps in meaning with 書呆子 (e.g. studyholic) and others describe a kind of 霸 that is not necessarily to do with 學習 (e.g. high/overachiever).