Perhaps:
There was a tennis tournament at my school, and I won it.

Interested in Language
Is the following sentence correct and natural?
I won the champion of the tennis at my school in 1999.
An English learner.
Perhaps:
There was a tennis tournament at my school, and I won it.
Not a professional teacher
Or, "I won the tennis championship at my school in 1999".
Or, "I won the champion trophy for tennis at my schopl in 1999."
I am not a teacher or a native speaker.
I am not a teacher or a native speaker.
I am not a teacher.
I am not a teacher or a native speaker.
I was the 1999 school tennis champion.
I won my school tennis competition/tournament/championship in 1999.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.