Do those 8th graders attend school on the same campus?
Are you a 4A or down school?
If so, IIRC, it's allowed.
As always - check with UIL and the C&CR for better information. I know there have been several rulings in the past few years about use of 7th and 8th grade students in high school ensembles. Unfortunately, UIL makes their website deliberately difficult to find information on, and "rulings" aren't included with the C&CR or with any policy documents.
There really, really needs to be a single-click place that lists ALL of the policy and rules interpretations that are currently in effect. Sort of like .. a rules document .. maybe even attached to the page that lists "contest rules" .. but since UIL is apparently run by nincompoops, it's not there.
(Don't even get me started on the places the UIL C&CR contradicts ITSELF or is deliberately vague. Someone really, really, needs to go through the entire website and policy-making part of it with a fine toothed comb, and has for as long as I've been teaching)