I am sure they have thought beyond S1
I think the show received an automatic 2 season commitment (some sources say three seasons) but I agree, the creative team has been working on where to take the show once this season's pentultimate mystery is solved.
If the Scoobies are in their senior yr then that means we could have them on the road for S2 solving mysteries. Either way, there are always plenty of mysteries to solve and charecter development to be had.
We know the gang aren't seniors due to the exposition from "Song Of Mystery", in which Fred's pint sized tutor WAS a senior. They are either sophmores or juniors. That's assuming that time will flow in this series to allow them to move towards graduation - as opposed to Bart Simpson who's been in the 4th grade for over 20 years!