There seems to be a Gemini kicking around on Gateshead locals, schools, spare in the Interchange quite frequently, therefore I would say 2 spare Geminis would be ok.
If 3 buses were off I'm sure putting an East Lancs B7TL or even a single deck on short Ten's wouldn't be the end of the world.
Another older bus would need to be brought in to keep the spare double deck numbers the same, assuming Riverside aren't losing any scholars in the next school year.
RE: Go North East: Upcoming Service Changes