The pythagorean theorem has many proofs, for your level we can opt of the simple geometric/algebraic proof.
If we construct a large square by joining 4 right triangles as follow in the picture, we will find the area of that square using two different strategies.
Area of each triangle is `1/2ba`
We have 4 of them so area of all 4 is `4*(1/2ab)=2ab`
The small interior square has (b-a) as measure of its side, so its area is `(b-a)^2=a^2-2ab+b^2`
Hence total area is `2ab+a^2-2ab+b^2=a^2+b^2`
Since the larger square has side c then its area is `c*c=c^2`
Both strategies give the area of the same square, thus
Hope this was helpful. There is a similar proof if we rearrange the triangles similar to the 3rd attached figure, I will attach a link where you can find that proof.
Good luck in high school.