Yes, you can say it like that. Most listeners will understand. We have no set method of asking or stating ordinals in rankings. Read [url=]THIS THREAD[/url].
There's valedictorian (highest GPA) and salutatorian (second highest GPA) but beyond that there are no specific terms. You'd be better off using best, second best, third best or highest GPA, second-highest GPA, third-highest GPA than "x top student".