Answer:
Don't join the points with lines
Make the numbers on the vertical axis larger
Label the axes
Explanation:
Make all the points different colours → I don't think that would be useful. Unless the teacher asked you to in class, it's probably not a good answer.
Put the title on the bottom → Same. Unless the teacher asked you to in class, it's not really useful and probably not a good answer.
Don't join the points with lines → Teachers ask indeed not to do this, so I think it's a good answer. Most of the time they prefer students to use curves (hope it's the good translation).
Make the numbers on the vertical axis larger → Since both axes represent notes, it would be easier to compare with the same scale. If that's what the teacher meant, then I think it's correct.
Label the axes → This one is interesting, because right now we don't what they represent at all.
Hope it helps, good luck for your exercices :)