Hi, I've got a bit of experience coaching kids in the middle school age range, and generally in groups of 50 - 75 kids, so I have a few things that might be of a bit of help for you.
First off, don't worry about recording times. Middle school leagues aren't that big, and kids will get used to seeing the same kids racing next to them in each meet. They'll measure themselves by how well they do with respect to their friends and rivals from their school and the other schools, and won't be particularly focused on the actual times.
Definitely don't add in any sort of time trial--improvement, even in the very beginning of a runner's career, is rarely linear, and seeing times each week can often be more discouraging than helpful. Presumably the kids have to run the mile in PE every few weeks, so they'll have some sense of progression.
I would also recommend against a repeated weekly workout. You're dealing with kids ages 11 - 13. While a few of them may be very serious runners already (you'll probably see one or two 5:00 guys and 5:40 girls on a middle school team of 50), most of them are there because 1) their friends are doing it, 2) their parents wanted them to pick a sport, 3) they thought it would be fun. You don't want to make the kids bored, and you don't want to scare them away from the sport. If you want consistency, have one or two days "long" days of 2 - 4 miles, a hill days with 4 - 6 short hill sprints, and, if you're really ambitious, maybe one interval day every week or two. Don't have weekend practices--let kids play soccer, basketball, bike, or whatever.
Focus on the fun aspects: include relay workouts that let the kids meet one another, pair up slower and faster kids to be teams during interval workouts, and have some kind of designated game day once a week (frisbee, capture the flag, chaos tag, soccer, etc). If the fastest 7th and 8th grade guys are clamoring for something harder, you can give them slightly more serious workouts, but at a middle school level, what you really want to be doing is building the foundational pieces for high school running.
Also keep in mind that the kids that do the training necessary to run around 4:35 / 5:10 as middle schoolers generally aren't the kids who win high school championships. High school championships go to the kids who didn't train a ton of miles in middle school, but put in the foundations to both train hard and truly enjoy running come high school. Ultimately, middle school racing doesn't matter that much, and the kids that are the top of middle school XC are often the ones that burn out by junior year of high school.