My kids were on the SAS waiting list and ended up getting into the school by July so I encourage you to put them on the list.
The first round of admissions is made in May when the school determines who will be leaving for next year. After that there are still some spots that open up since many will keep their place even if there is a possibility of a move.
Review the way their priority works – 3 years ago if you accept a Pudong spot you moved to the top of the wait list for Puxi. Also once one kid gets in the other moved to the top of the list.