GMail and SPAM from my professor

I am using GMail since 2005 and for the last four years it did an awesome job of keeping me clear from SPAM. I've had months where not a single SPAM message was delivered. Now, suddenly, I am regularly finding some SPAM in my inbox, most of which has the email address from one of my professors set as the sender. At first I thought, he might have caught a virus that's abusing his computer, but that would have stopped by now, so I think that the spammers are just faking his email address. Oddly enough, the university's spam filter marks the message as SPAM, but GMail doesn't filter it out.
I think here's why: the SPF sender authentificationn declares the sender as permitted, so GMail (rightly) assumes that the professor is really the sender, therefore the mail will not go to the SPAM folder. I think this is right: email from somebody who has sent me many non-spam messages and whom I even sent some messages back should always be trusted. The problem seems to be that some spammer has managed to send SPAM from within inside the university.
I will ask our admins to look into this.