You probably will not have trouble checking in the night before but I cannot say that I've tried to do so out of YYZ. Toronto is not on the list of airports that will not accept bags if more than 4 hrs before your flight. Also not used to how CATSA will handle this vs TSA.
If the last flight is 10 pm I would err on the side of caution as the checked bag cut off for international flights on AA is 1 hr before the flight so if the check in desk closed around 9 pm I would not be surprised.