Koays pretty much nailed it in brief. I'll present the TL;DR version.
Kitty joined before the New Mutants were formed. There were no junior students training at the School, so she ended up joining the X-Men because she wanted to help them. By the time the New Mutants were formed, she was too experienced and felt that she had proven herself as an X-Man already. After proving her case to Prof. X, she was allowed to stay on the senior team. You should read X-Men Vol. 1 168 (the famous "Professor Xavier is a jerk!" issue) since that pretty much wraps up the situation, and should be easy and inexpensive to find, though I liked the New Mutants issues where the junior team felt annoyed that she didn't want to train with the "X-Babies."
As Koays said, Jubilee was a stowaway; she followed the female X-Men home after a shopping trip, and then hid in their headquarters - she joined right after the Dissolution storyline ended with the X-Men going through the Siege Perilous and losing their memories shortly after Storm got "killed," and the Reavers attacked. Storm was actually de-aged and kidnapped, ending up in Genosha with Gambit. Jubilee saved Wolverine from possible death (he was crucified and his healing factor was malfunctioning), the only X-Man left when the Reavers attacked, and she became a de facto member via virtue of being Logan's partner. The storyline at the time had a bit of juxtaposition going on. Jubilee joined when there were no X-Men, and she rescued Wolverine. When the X-Men re-formed, there was, once again, no junior team so like Kitty, Jubilee ended up being assigned to the Blue Strike Force. At the same time, Storm in a child's body brought Gambit into the fold, and when the X-Men reformed and she returned to the correct age, he remained with the team. Jubilee typically didn't go into the more dangerous situations, though she often ended up following the other team members, anyway. Same treatment happened to her - as soon as the Massachusetts's Academy was re-opened and Generation X was created, she was sent there with a lot of Kitty's similar feeling, but with less maturity to back it up. This was another juxtaposition because the Massachusetts's Academy, in an alternate future where the White Queen kidnapped Kitty and captured the New Mutants during their rescue attempt, it was shown that Emma Frost had quite effectively melded the teams into a united class - kind of similar to what she did with Gen X, though she didn't have to kidnap anyone to accomplish it. Didn't want to get sidetracked, but it's worthwhile, IMHO, to note where Kitty and Jubilee's stories are similar, and how the overall arc of how the training aspect of the X-Men's junior teams ends up as history repeating itself.
Rachel was older and much more experienced, so she was in a situation similar to Dazzler, Psylocke, and Rogue, not Jubilee or Kitty. I believe Rachel was about 18 years old when she met the team, and she actually wasn't an X-Man for a few issues. Comparatively, Karma was the oldest of the New Mutants at about 17-18 (but she wasn't around when Rachel officially joined), while the next oldest was Sam at 16. Kitty was about 16-17 when the New Mutants were formed (but had joined the X-Men when she was around 14), while Jubilee was 13-14 when she formed. When Dazzler first approached the X-Men for training, she was likely in her early 20s, and while she appeared in a few issues of the New Mutants, there was never discussion about her joining the New Mutants, likely because she was already skilled with controlling her powers. Psylocke was in a similar situation - there was some talk of her joining the New Mutants, but she was already in her 20s, and the discussion never materialized into anything before the Marauders struck. Rogue was also a similar age - 20's, but her issue was that her power was uncontrollable and I believe there was never any conversation about her joining since she was considered untrustworthy - after the Marauder attack and Storm's decision to abandon the mansion and hope their enemies stayed away from the New Mutants if they thought the X-Men were dead, there was never any mention of this again had Rogue had proven herself. In regards to training, Rachel was already quite skilled with her powers - there was nothing for her to appreciably learn. The New Mutants didn't just learn to control their powers - they were very much modeled on a boarding school for junior high school to collegiate level courses. They had classes, and extra curricular activities in addition to combat training and power development. Older members would have presumably been taking collegiate courses if they hadn't attained some level of education already, and if they had already done so and just needed to learn to control their powers, those training sessions could happen separately. So there was a rational, story basis for why new members past a certain age threshold were considered X-Men trainees rather than New Mutants.
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