Because when you multiply a negative number with a positive, you are multiplying the negative number with a positive value. I don't know how to put it to a secodnary school student, but think of possitive as "love" and negative as "hate".
When you do positive*positive then you love to love. wich means that you love. If you do a negative*positive then you hate to love, which means that you hate or that you love to hate. which is still hate. But when you multiply a negative by a negative then you are saying I hate to hate, which means that you love. hense -5(-5) = 25
Imagine you have a sqaure. It can have a length of a= 5m. The area would be A = 25cm^2.
If you have a negative length, it simply means, that it goes in the opposite direction. So you get the same square, it is just upside down. This means it still has the same positive area of -5*-5= +25
