First of all, why are there only 6 students in the class. Second, how do TWO students forget their lunch.
Anyways, the answer is 1/15.
The probability of picking a student who forgot their lunch the first time is 2/6 or 1/3. If you picked one student who forgot their lunch, there is 1 student who the teacher still needs to pick out of 5 students. So the probability of that is 1/5. Multiply 1/3 and 1/5 together to get 1/15.
The teacher has a 1/15 probability of picking both students that forgot their lunch.