I'm working on sort of a "thriller lite" with sci fi twist, but the characters are graduate students. Will this work?
YA is more about tone than anything else. Most, but not all, characters in YA are under 18, but people still sell stuff about adults as YA, though less frequently.
A good example is Mal Peet's Carnegie Medal-winner Tamar. Admittedly a third of it is narrated by a fifteen-year-old girl, but two thirds of it deal with characters in their twenties. It's a wonderful novel, but I do wonder if it would have been published as an adult book if Peet hadn't already been published as a YA writer.