In the 152, take off with no flaps, full power, full mixture. Pitch up (rotate) at 65 knots and try and hold 75 knots in the climb. It’s an underpowered airplane, so you should try and be as precise as you can with your airspeed in the climb. If you’ve got the nose up too high, speed and thus climb performance can drop off pretty quickly, and you can wind up without enough power for what you’re asking the plane to do (aka “behind the power curve”).
If that doesn’t work there’s something else at play. But I can’t imagine they set up the training flights in locations where you would need to lean for takeoff, for example.