The numbers are small enough that, although it isn't required, you can directly enumerate the possibilities. This can prove enlightening.

The general idea is:

1. All possible ways to choose 2 grads out of those 4 grads is the sample space.

2. Find out how many elements (pairs of students) in that sample space correspond to having choosen 2 grads who intend to teach high school.

Though these aren't the numbers for this problem, but suppose the sample space had 500 pairs of students. And suppose what you were seeking corresponded to 100 of those possible pairs. Then the probability that you found what you were seeking would be 100/500 = 20%.