I think you must describe what kind of proofs your class is doing.
There is no single standard for what constitutes a mathematical proof. Proofs written for math journals are a different matter than proofs written as exercise. Proofs written as exercises have standards that vary with the instructor and the course material. For sake of ease of grading, many classes want proofs written in a very concise, barely literate form. In contrast, a few instructors want proofs written as essays using complete sentences, correct grammar and correct spelling.
I'm trying to write some advice about doing proofs since I promoised micromass I would post one. It would be helpful to me to discuss the details of your students' problems.