What would be the point? What was the point of anyone's infestation? She was a host, she was raw meat to the Yeerks, a pork chop, a hamburger, she was nothing to them but a consumable, why infest her? The better question is with morph capable humans and lots of hostless Yeerks and the deal with Chapman no longer necessary why
not infest her? A young athlete, good physical health far as we know, already mentally damaged, throw in morphing capability and a loyal Yeerk and you've got yourself a frontline soldier the likes of which Rachel would have the slightest of hesitation killing. Why leave her, a friend of a known Animorph whom said Animorph might contact, free to make said contact and endanger a reasonably important Yeerk when she could have been used against the Animorphs instead, might even have been used to lay a trap for them? I'll actually grant that the fact that she never was used that way might indicate that she remained free, but it could also be that a proper chance never came up.
The objective is to infest as many humans as possible, Visser Three/One showed long before that he didn't care about his deal with Chapman so what exactly happened to her when there was no secret about the Yeerks? Both her parents were controllers, there was no one to take her away to safety, so either she was infested or she was on her own, how normal could life have been for her then? How much more dangerous with two controller parents? She had value as a host she was worthless to the Yeerks as a free dependant, worthless things get discarded. Chapman was only a host, whose only weapon had disappeared when no parents would actually show up at conferences to see him wig out in public, Iniss 226 didn't even care about the girl, he wouldn't have kept her as a pet the way Visser One planned to keep her kids.
Now does all this mean that she was definately infested?
No. 'Course not. Maybe she was just so mind boggligly unimportant to the new Visser One that he never said "Melissa Chapman, you win an all expenses paid trip to tour the stars as a shock troop of the Yeerk Empire, bring that lucky kid right on down for infestation!" maybe over the course of the war Iniss 226 developed a sort of fondness of Melissa and sent her away to a non-infested relative before the pool ship descended, maybe they just kept up their charade with Melissa being even more confused as to why her parents didn't worry about the big alien space ship shooting down fighter jets and blowing up buildings. My word is far from holy script and it can be argued with but for now it remains the more likely and realistic explanation to me personally, that Melissa was infested at the end. The pros to the Yeerks outweigh the cons. That however is not the subject of discussion here, though if we want to make another topic dealing with Melissa's possible infestation that'd be fine with me.
Actually I kind of wonder sometimes if she would have been infested much sooner, she fades into the background so completely that she could have married a Hork-Bajir and we wouldn't know it--no, stop discussing her, bad me, bad!
Anyway back to Chapman, if they had escaped why would he send her to another school? To keep up the illusion of normalcy? Well obviously she wouldn't go as Melissa Chapman, and again if they slipped under the initial net of the Yeerks, with Chapman not being that important in the grand scheme of things they wouldn't send bug figters after him, wouldn't be able to send very many police after him since they would only be able to rely on controller-cops, again a group not every police officer belonged to. The Yeerks would instead wait to see who starts talking about space invader slugs, when Chapman didn't they'd figure he wasn't a threat anyway, again they couldn't find Loren even after they had the link of Elfangor (or Al Fangor) and Tobias (Fangor?) they never (to our knowledge) got the guy from the park in book 3, they even (for reasons totally beyond me) never seemed to figure out that Karen had stopped showing up at they Yeerk Pool or that Aftran wasn't infesting her anymore. The Visser was far from omnipotent, escape was very possible and assuming Melissa could be reasoned with staying away and safe was also (Slightly less but still very) possible.