I varies from school to school and organization affiliation, etc. The ones I've seen are pretty much the same in that they line up (most in rank order, although not all), the teacher comes in and meditates a short time, claps two times, four times, or not at all, bows to the shomen, while the senior student says, "O-Sensei rei," (bow to O-Sensei) and as the teacher turns to face the class and bows again, the senior student may say something like "Sensei ni rei" or "Shihan ni rei." In some schools (as in ours), when the students bow to O-Sensei they say "masakatsu agatsu" and then for the second bow to the teacher say "onegaishimas." At the end of class we repeat the same, except for the second bow in which we say "domo arigato gozaimashita" and the teach responds "doitashimaste" (sorry Jun I know my spelling's wrong on that one).
Seriously, AikiWeb's files on this type of stuff are really good, and probably more literate than my explanation. I also agree that you should ask the senior student.