I think Jason could be A because we can notice through this season and since the beginning that A is especially against Hanna and Spencer who are the principal and favorite victims of A. In that case, Jason seems to be the principal one who has enough reasons to take vengeance after them.
In the case of Spencer , we can imagine that Jason wants to take vengeance of Mister D who is also his real father who abandoned him and for having never considering him like his real son and otherwise he hates Alison and Spencer .
For Hanna , we can imagine that now that Ashley rejected him he wants to take revenge by mistreating Hanna .
Guest theory submitted by @MissMonteith78