A school has $180$ students and $3$ teachers. In the first period, each student is taking one class, and each teacher is teaching one class. The enrollments in the classes are $90,60,$ and $30$. Let $t$ be the average value obtained if a teacher is picked at random and the number of students in their class is noted. Let $s$ be the average value obtained if a student was picked at random and the number of students in their class, including the student, is noted. What is $t-s$?