Jack Russell's are tough, big hearted dogs and nobody told them they are little!!! They just don't believe it. If you want to stop this behaviour you will have to imagine her as a GSD. Allowing her to jump on you is inviting her to treat you exactly as she wishes.

No more jumping on laps until she has been calm for at least ten minutes - that is relaxed and lying down away from you - then only when invited. When ever she nips the best way to get the message across is to yelp,"Ouch!" loudly and turn away from her for 5minutes. She then has to sit nicely when asked before you resume contact. If she pesters you when you are ignoring her, stand up and DO NOT NOTICE!