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