call the YM office of your diocese, or Catholic schools office, they may already have retreat teams that can come into the school or to another location and run the retreat part, and the parents would just have to chaperone, get supplies, do the food etc.
I don’t recommned a sleepover for this age group, but consult your diocesan guidelines if you do plan one. Our 8th graders from the school go to a came N. of San Antonio for an overnight retreat, but it is pricey, but the price includes everything except the charter bus.
I think a weekend day works best, not a school day, and tied in with one of the regular Masses. We usually have our Jr. High retreats on Sat. and end with the 5 pm Mass (confessions beforehand), and HS retreats on Sunday beginning with the regular “youth Mass” at noon, going until 6 (late enough on a school night). Sometimes we plan to coincide with public school vacations, but then we lose a lot of attendance.
Pick a theme.
are these 8th graders confirmed or preparing for Confirmation? that is a natural theme. This age group is very interested in practical applications of their faith, discipleship, service, so building a service project in as part-- a couple of hours, no more–of the experience might work. What does not work with this age group is the format that most adults like: talk, reflection, prayer, talk, reflection, prayer. They need a lot more process activities to drive home the theme
pick one theme and stick with it, one idea you want them to come away with.