Are you teaching in a private school? Are there aids? This will give me a better picture of what you are working on.
There is a network of sites linked to the one above that has the philosophy and practice of being inclusive of all students, from the weakest to the strongest. It is not a traditional approach. There is no textbook, no simple syllabus to deliver. But that is the whole point because no single textbook/syllabus can meet the needs of all the kids in your care.
If you are in the US, you have the summer break to study the the sites and decide how you could use the materials in your situation.
I feel your pain. I, too, work in a middle school and am dealing with students with widely disparate ability levels. It's a constant challenge, and I often feel as though I'm in a if-you-try-to-please-everyone-no-one-will-like-it situation. I do, however, have a couple of pieces of advice:
First of all, if you have supportive administration, enlist their help. They would be the ones who can get you teacher's aides, resources, time, (i.e. prep period instead of lunch duty,) etc... Your Child Study Team can help, too. Are these low performers Special Education students? Should they be? (If a student's actual performance is more than 1 and 1/2 standard deviations away from what their testing results would indicate, they should be classified, and then they (and you) could get all the assistance that goes with that designation.
Secondly, and this is again assuming that your administration would be supportive of this, don't be afraid of the curriculum police. Work with your colleagues to define exactly what these kids need - and I'm talking bare minimum - and then build up from there. Sometimes we teachers fret about doing as much of the curriculum as possible, thus things can sometimes get passed over without a solid understanding of the concepts. At that point, everyone is just spinning their wheels, so why not step back and fill in those gaps?
And while you're filling in those gaps, have the high-achievers work on some open-ended projects, or units, or learning centers. It's tricky, but when selected carefully, those kinds of activities can do a lot towards addressing student needs. Weirdly enough, the nsa.gov website, (and yes, that's the National Security Administration website,) has a section of really solid units, broken down by grade level. Why the NSA is developing K-12 math units, I can't even fathom a guess. Maybe it's better not to ask those kinds of questions... But seriously, check out the nsa.gov website - there's a 'Commitment' tab, and inside there is a link to a 'Promoting Math and Science' section.
Ultimately, if you have put in the time to develop a solid, disciplined, and truly safe learning environment, the kids will be much more receptive to the process that goes along with woking with kids at different levels.
Good luck, and keep up the good work!