1. Check your uni's regulations around that. Mine required companies to set an expiration date after November 1st, and to give all offers at least 1 week.

2. I'd just tell them you need more time (1-2 weeks minimum). Recruiting is expensive- if they really want you then they will be flexible regardless of "company policy". It's not worth it to lose you and have to interview 10 more candidates. This works better for smaller companies, the large ones tend to have too many applicants to care.