I'm going to make the assumption that we are dealing with a child whose age is in the single digits.
I think your immediate problem could be solved using something like this:
http://www.guardianangeltech.com/product.htm (although I suggest further google searching, it looks like you are not the first to encounter this problem).
You could give her the cell phone and encourage her to treat it like a pet, she needs to learn responsibility eventually. It may even be useful to put a virtual pet game on it. You don't need to explain to a 4-10 year old what the true purpose of the device is.
However, this should be treated like a last ditch safety net, and should also include all the traditional things, make her memorize her home (or your cell) phone number, your address and how to get help if she is lost. After all a cell phone can (and will) get lost.
You could (or should depending on how assertive you want to be) also try to look at how the school bus line up works these days and see if you can help your school come up with a better way. Chances are no one has had the time to really sit down and come up with a simple wiz bang way to deal with all the kids getting to the right busses. And just in the brain storming that is being shot at you here, you probably could show up with a list of things the school may be willing to try (especially the simple cheap things). Some people may appreciate it if you try to help them improve, rather than just pointing out how crappy they are.
And the final stage is the most complex part. You will need to talk with your daughter (when she is older) about what you did and why, and even why what you did may have been wrong.
And be certain to cut the leash. Not when you are comfortable, but when she is comfortable... You will NEVER be comfortable, not even on her wedding day. And recognizing the anxiety early and learning to control it, not ignore it, and that will help to make a great parent.
Good luck, and know, if this is the worst bump you've run into so far, you are doing dam fine job.