Is there a word for 学霸 in English? 霸 comes from 霸王， which is an attribute people gave to a super warlord in ancient China who is invincible even when he was just fighting along against an entire army. And they would use this term for people who are academically dominant.
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).