There are 18 girls and 12 boys in your class. what is the probability of a new kid entering your class will be a boy?

Let's assume that your class is a random sample of the school's students as a whole.  In this case, the probability of a new child entering your class being male is the same as the probability of randomly picking a boy when you select a random student from the class.  There are 12+18=30 students in the class, so the probability is 12/30=2/5.

Note that without this assumption, the probability is indeterminate.  However, given that the question is listed under elementary school, I assume that they are expecting that assumption to be made.