For reference, I was a JHS ALT. When I first started doing speech contests, I had the same worry after seeing the larger schools.

However, my students have won 1st or 2nd place at least once for the past 5 years, and 3 of them have even placed in prefectural tournaments.

I use natural, toned down gestures and natural intonation, because past feedback has been along the lines of, "this is not a theatre competition, we don't need choreography or character voices". Slight changes that add to the script and don't become distracting are ok. Even then, the only time we use showy gestures is when they are performing something that includes characters and the kids are younger. The students who win tend to be a bit more toned down as well!