If you think your sons are gifted, you can talk with the school guidance counselor and ask to see him/her in person to discuss your request. Every school has a committee focused on students with special circumstances, including gifted students. That is the first place to start.
I might caution you that testing students in high school is a bit unlikely because high schools offer a variety of levels of classroom subjects in such content areas as English, mathematics and science.
At the same time, if you feel strongly about this you should go ahead and meet with the counselor as soon as possible.
To be honest, 16 is to old to be tested. There is no benefits of the testing at this age. For the younger kids, they can be tested to be enrolled into gifted programs, and they kan be tested in the last quarter of K.