Use an active not a passive voice.

Use details, but don’t get wordy.

Use correct punctuation to make your writing clear.

A few (certainly not comprehensive) suggestions:

As a student in your class, I often needed to clarify questions arising from one week’s lesson before the next class session would begin. Because you had set office hours, I was able to come to you when I needed help…

Now explain the teacher’s reaction to frequent requests for help and how the help made a difference.

2. Look over your paper a couple times to change little things you see at a second glance.

2. Have a friend (or two or three) edit your paper.

3. Have another teacher edit your paper.