Terry, you did a good thing getting the maths book from school.
We went through this a lot with difficult child 3. It probably began as long ago as Grade 1 or 2, would be worse at the end of the school year and it only took me four years to finally get the message that the problem was school anxiety/school phobia developing/escalating primarily due to bullying, an inability to cope in a mainstream setting and even teacher bullying.
It did begin as you describe. And I was fully aware tat difficult child 3 gets into habits FAST! If I ONCE bought him an ice cream at the shops while shopping, he would then expect one every time. In the same way, if he took a day off school by being sick (or seeming to be sick) and he had a more plpeasant day than he would have if he had gone to school, then he got an immediate positive payoff, whichc made it more likely he would do the same thing again.
Now, difficult child 3 was a kid who honestly wanted to go to school. But he felt sick, genuinely sick. And it was all anxiety, but nxiety which I ignored (cobination of probles, including not recognising it was so extreme) until he really WAS physically, ill; he would be vomiting at the mere mention of school. This took years to develop to the severe levels we had, but I had no help with him and supporting him/reducing the anxiety in that time.
I could never be sure if difficult child 3 really had a virus, or was simply shamming or talking himself into being sick. Ofgten he would actually have a low-grade fever, as much as a full degree C above normal (which would be 2 degrees F above normal) so the thermometer was no guide. So by Grade 4 (and especially in Grade 5) we had put in rules.
The first BIG rule - SCHOOL WORK DURING SCHOOL HOURS.
Now, difficult child 3 was a very bright kid academically, especially in the harder subjects. So I was horrified as I watched him do schoolwork (we began on homework while I waited for the teacher to collect a package to send home) and realised how little he knew, especially in subjects like Geography. He also had never been given extension work in Maths as I had requested, so I went out and bought resources to supplement what I was getting for him to do from the school. That way I never ran out, I always had something for him to do. I had some computer educational software that I let him play with, once he had completed what I felt was sufficient written work. But it was head down, tail up time for the entire school day. If he felt well enough to sit up in bed, he had book work to do. If he didn't feel well enoguh for book work, he could read a school book or watch an educational documentary on TV. If he was feeling too ill and was sleepy, he could get out of work by sleeping. Because I knew - he would only ever sleep in the daytime if he was REALLY sick.
In this way, I was trying to prevent him bulding up a Pavlovian response to being at home on a school day. Even at a subconscious level, if he got any kind of reward for being home, then he would increasingly feel sick on a school day.
The end result - we finally realised he needed to be at home (in our case). But by that time, difficult child 3 had done a lot of work at home which helped him catch up academically, on some really nasty educational deficits. Otherwise he would have started high school still not recognising the shape of any country (including our own) on a map. We'd go on holidays and he would ask which country we were driving to (not possible, in Australia). He had no idea of north, south, east or west, before we began this. he had been exposed to it at school, but had simply not taken it on board, and nobody had noticed.
So this rule above all others, I heartily recommend for any difficult child home sick (or allegedly sick) from school.
If the kid is consciously "swinging the lead" (called plumbopendulitis in our family) then he won't be able to get back to school fast enough, after a few days of the "mum school" treatment. Not that I was ever nasty or anything; I just made sure that there was no play, no entertainment that wasn't also educational, during school hours. And of course, a kid who was too sick to go to school is far too ill to go play with friends after school that day.
Marg