Yeah, students in my classes often run into obstacles in trying to make what they want too. I think that this is a good lesson in the realities of programming and just life in general. How much time and effort are you going to put in to get the desired result... The really motivated student may come up with some really creative solution where other students may tend to give up easier.
One way that I can think to do this is make the obstacles out of more than one one sprite, add them to a group, and then say, if group is touching the player's sprite. That way you can break down the the triangle into a series of increasing sized rectangles. (I'm not sure that this makes sense.)
I'm not sure there is a lesson that deals with groups yet. You can find some documentation in the game lab by selecting group in the toolbox and then hovering over the different functions and whatnot in the toolbox.