He is such an eager beaver at school.
eager beaver
idiomOne (especially a child) who is very excited or enthusiastic to begin a task; a person who is exceedingly assiduous in an enthusiastic manner
Origin
disputed origin*
The new intern is an eager beaver who always arrives early to finish his tasks.
While the rest of the team preferred a slow start, the eager beaver had already completed the entire project proposal before the morning meeting began.