A good topic - and so far hasn't gone down a super negative path like sometimes happens here!!
I coach a team of 40+ girls and we have a wide spectrum - some proper runners, some who like running but maybe aren't that gifted, some who are doing XC to get PE credit and some who just are there for the social aspect. Lots of high achievers as well, involved in other activities like Student Council, club sports, band, leadership programs, etc. which sometimes take them away from practice or races. My goal as their coach is to field as competitive team as we can without risking injury, but more importantly to turn them into runners for the rest of their lives for all the benefits I've seen from running (physical and mental health, confidence, camaraderie, etc.). We've had some really good years and some not so good - but like all of you have seen in your running if you show up consistently and give effort you will see progress, and we've seen that.
Our rule is 2 unexcused absences before you miss a meet, and I have the discretion to what is excused versus not...I am pretty reasonable but I am firm that they must communicate with me when they are missing, why they are missing, and we agree on the implications. I also make it clear up front that if they do have competing priorities with other activities, and it winds up impacting how much they're able to race, there are no hard feelings and I don't think less of them and I'm not mad at them. Part of being that age is that they need to learn how to communicate with an adult. Unless they're being deceitful or doing something they shouldn't, they should be comfortable enough to approach me and say what's going on. It's pretty straightforward - they know the potential consequences of missing practice, they have a choice, and if they choose the other activity and have to miss an extra meet or 2 that's fine by me.
We also give every runner at least one meet off per season - they can fit in other things around that, it helps reduce injury, it keeps them fresher, and it can always be that meet that they are being "punished" for missing practices. It's pretty rare that a top runner prioritizes other activities over running, so I explain to the girls who do have to miss from time to time that they may be able to race, but maybe not Varsity or JV (thank heavens for Open races!). Getting a spot on Varsity or JV is a privilege to be earned and the person with better attendance is going to get that spot over someone who is a little more spotty.
Once the girls have gone through a few weeks, we (coaches) know who is committed and who isn't. We know who is really battling some aches and pains, and who is just dogging it. Each girl is assigned an individual coach who sits down with them weekly to review their log, talk race strategy, and check in with them. It's pretty apparent who is into it, and who is there for other reasons...and that's OK. We focus on the ones who want the coaching - the others we don't nag, we guide them but we don't babysit them, and several of them have actually turned into pretty committed runners after a season or two BECAUSE we weren't making it a miserable experience for them.
XC is different than other sports - as long as someone isn't being disruptive at practice, or dragging down the effort level of others, if they show up and give a moderate effort they won't see the results they might otherwise but that's their choice. They won't take a spot away from another runner who is more committed so focus on who is there and working hard.