Answer:
With a little more focus on classroom tasks, she can do wonders.
Step-by-step explanation:
When using the prepositions on and in with a noun, on is used for people and things/objects, whereas in is used for places.
To use in instead, you could rephrase the sentence to say:
"With a little more focus in the classroom, she can do wonders.
This changes the noun of the sentence to be a place, the classroom, instead of a thing/object, the classroom tasks.