Always review the material you learned in class when you come home. In fact, it would be beneficial that you stay one day ahead of the class. That way, if you have any questions, you can ask the teacher that day. Always start studying one week before a test so you won't have to cram 2 weeks of material into a single night or two. For exams, at least 2 weeks since you are covering 4-5 months of material. The earlier, the better. Instead of pulling all night sessions, your anxiety will substantially decrease when starting early.
Always encourage your kids to do the best that they can. However, you don't want to be overbearing. That can have a negative effect on them. There's a fine line between encouragement and pushing a little too much.