Your server is probably not active enough for segregation of low and high lv. So it's either that or less than 10 ppl on each team, which frankly does not end well (people gets kicked out faster when there are less people around, and that reduces the ppl on each side further, potentially ending with only 2-3 ppl one each side, highly repetitive fighting by the end)
There is really no good solution to it other than server merger, which is incredibly slow going.