Are there alternate ways to meet Jack in Mass Effect 3?
I waited too long and the Grissom Academy mission went away =(. Is there any other way I can meet Jack in ME 3? Or was that the only chance?
No.
Well, that's not entirely true. You'll see her again, but it won't be friendly.
If you didn't do the Grissom Academy mission, Jack will appear in a later mission as a Cerberus Phantom. Aside from using her trademark taunts, she will be no different from any other Phantom.
This is similar to how Morinth will appear as a Banshee unless you killed her during Samara's loyalty mission in ME2.