Paragraph
When I was a teacher, I showed my teaching skills. I taught 50 students.
I am good at explaining maths.

sentence 1
When I was a teacher, I showed my teaching skills. (past tense)

sentence 2
I taught 50 students. (past tense)

sentence 3
I am good at explaining maths. (present tense)

In my opinion, it's ok to use more than one tense in your paragraph.It all depends on what really you want to emphasize.

Here, you use the present tense in I am good at explaining maths.

        Present tense often refers to the fact and the truth ( your ability), and the fact last forever ,it (your ability to explain)won't change with time.

If you use past tense --> I was good at explaining maths. Past tense usually refers to the past instead of now and continuous condition. If you use past tense here, it means that you was good at... before, but now you are not good at and you lose this kind of ability to explain well in maths.