Esther your intentions are good, but there is no reason why really excellent students should be held back because slower-learning students cannot keep up with them. And Hashem seems to teach this idea to us in the halachos of kilayim. An ox and a donkey cannot work the same plow tethered together under one yoke (because the ox is much stronger and will tire the donkey out too quickly).
The classes SHOULD be divided according to the strengths of the students, but IMO NEVER referred to as an Aleph or Beis (or Gimmel,etc.) class. And to ensure that all kids play together, the class recess shoudl always be together ina spirit of achdus, class plays, siddur parties, etc. should always be done as a group where possible. No one should be singled out. BUT neither should a bright child who “chaaps” things quickly, have to stagnate while waiting for others in the class to catch up. Giving extra work is not the answer. The child feels not-so-blessed for being given this “gift” of enrichment. Put him in a class with kids who are on the same level, unless there is no breira. It’s also nice that you want the brighter students to help the ones who need a little extra, but that is also an unfair onus to put on a student. ALL kids should always be encouraged to help each other, not just the brighter ones.