Have we all forgotten about Metering?
If its a normal stage where the stage is lit up bright and the audience is in Darkness - you should set the Metering mode to SPOT or PARTIAL metering.
If the whole place is lit up, leave the metering to the most general one: EVALUATIVE.
Other than that - ISO 3200 - BUT I dont know if the Rebel has an expanded ISO where you can se it to 2000 instead of 3200, if its too noisy.
Based on the OP's question, I don't think metering was ever mastered much less forgotten. If the OP is using the 55-250 on a crop camera, the venue would have to be as big as Yankee Stadium for that lens to not be able to fill the frame with the 'stage', but yes you do bring up a good bit of information for knowledge building. If the OP has parts of the dark audience in the frame, then spot or partial would be better.
Also the T3i has the same sensor as the 7D so it can go to 6400 without turning on the 12.8K custom function, but I did some reading up on the t3i. With the pop up flash, the camera automatically sets the ISO at 400 in basic modes. That means the max subject distance is about 15 feet from the camera for pop up flash when using those basic modes.
When in sports mode, the camera can auto select up to ISO 3200 if needed, so if that is still giving subject blurred images, then the OP is getting close to being maxing out with out getting a faster lens. ISO 6400 and higher isn't going to give very good results regardless.
Another thing the OP should do is check the setting for max ISO if using auto ISO to make sure it is not set at 400. Max it out at 6400 or at least 3200.