Finding the percent of calcium carbonate in commercial antacid by using back titration
In chemical analysis usually do three times the same experimet, so you can minimize experimental errors.

After that, in order to find where is the error you can check:

Has everyone in you class use the same HCl and NaOH solutions (taken from the same bottle i.e.?

The antiacid tablets are all the same (given by your teacher)? If don't, could you post the composition?

Have you measured correctly the volumes used? To obtain that 210%, it seems tha you have used around 25 mL of HCl solution instead 50 mL.