Studybug does not seem to mean someone who gets the highest (or even high) grades.
Straight A student could work. In the A/B/C/D/F grading system, the highest possible grade is an A. So achieving an A would be achieving the highest grade.
However, if someone got an A, that does not mean that the person earned the highest scores on exams and assignments (in the class; among the other students, etc). For that, you could frame it in terms of class rank and say
at the top of one's/the class
: having among the highest grades in one's class
<He graduated at the top of his/the class.>
So if you have a friend and he in fact scored the highest marks, then you could say he was at the top of his class, or simply he was top of his class.
In terms of percentile, could say he scored in the 90th percentile, or simply he scored in the top 10 percent.
If you use of his class, this can be interpreted as his graduating class. So if it's not clear from context, then you might need to be specific and say he scored in the top 10 percent of his Algebra class, for example.
You can swap 90 for other numbers like 95, 99, and swap 10 with 5, 1, etc.
I might have taken the description given by OP a little too literally. In any case, if you are looking for words to describe someone who, broadly speaking, studies often and regularly gets good/high marks, then there are several options, but none seem to be exact translations. In this case, studybug and straight A student seem to work.
You could also try:
an unstylish, unattractive, or socially inept person; especially : one slavishly devoted to intellectual or academic pursuits <computer nerds>
This does not necessarily mean the person gets high (the highest) marks. Since nerd could be considered "bad", you can use nerdy to soften it. She's so nerdy. She loves reading his math books. (It's kinda cute, in my opinion.)
: a person unusually devoted to reading and study
This does not necessarily mean the person gets high marks, but it does mean the person studies a lot.
a person who does more than they are expected to do or who is more successful than others:
In an academic setting, such a person regularly goes above and beyond what is expected and gets high marks because of it.
Here are two references:
- English equivalent of two popular Chinese slangs...
- Is being a xueba (学霸) a good or a bad thing?