It is not completely necessary to screw your bathtub directly to your studs. While we do not recommend drilling through your flange, as this may void your warranty, you can still secure your tub. You can drill into the wall stud directly over where the flange would rest. Then, you can insert some washers and bolts to "sandwich" the tile flange between the stud and the washer. This will secure your bathtub to the studs without needing to drill through it.