Dear student,

First of all you must mention the college name. Now, the insider of any college is that - many of the teachers or faculties are on contract baisis.
Or if some college has scarcity of facilities then the hods or professors send the students to teach who are doing Phd. And then they will take their class. And if the contract of any faculty is over then he/she will left the college.

Now, the mathematics ks permanent or not, then you have to enquire this yourself in your college. Because you haven't mentioned the name and even if you had mentioned then also my answer will be same, i.e, what I have told about insider of any college.
So, you must get in touch of your lower faculties ( not HODS or professors ) so that to an information.