1. High school students are high school students, and only the top two (or three at max) actually get lessons and play tennis competitively. The rest are learners or casual players. Thus, they don't have the physical stamina to play singles and then doubles.
Train to get the stamina. Doubles warms them up well for singles. They will all take it more seriously and it will push their fitness level as well. Time is an issue, but matches will end when a team has 4. In college most matches in tournaments go 4-0 or 4-1. The rarer longer matches will be exciting as tension builds.
Southern California Tennis Academy