^Instores, I wasn't able to stack my 15% student discount on items with sale items that had additional 20-30-40% off but I was able to at other stores. So I think it depends on SA. Not sure about online though.
JCrew policy is that you aren't supposed to be able to use the educator/student discount on anything other than full-priced items. Additionally, it's not supposed to be used in conjunction with other coupon codes. Of course, JCrew policy is never consistently enforced (i.e. - people being able to get priced adjustments on sale items, etc.) so it really depends on the SA.