McGraw Hill
Glencoe: Writer's Choice: Grade 9 Proofreading Practice
This site provides 2 exercises in which students proofread passages for errors in spelling, capitalization, punctuation, and usage, and then retype the passage correctly. When finished they can click on Answers for feedback.
Other
Spelling police.com: Proofreaders' Marks
A general list of marks and symbols a proofreader may use in correcting and editing a piece of written work.