Calculating student improvement rate fairly
I am a teacher and I am trying to compute student improvement on a standardized test. The problem is best illustrated through an example. I have a student who had a 90% on a test last year, and now has a 99% - a 10% improvement. Another student scored 20 on the same test last year, and this year scores 24 - a 20% improvement. I believe intuitively that the student who had a 90 now achieving a 99 is more improved than the student who had a 20 and moved up to a 24. Yet the 24 student is showing twice as much improvement, based on percentage. Is there a more fair way to calculate improvement, or is my intuition simply wrong?