My youngest daughter definitely gets physical symptoms from stress which sometimes include headaches, backaches, and even itchy scalp and severe dandruff. Around 2nd grade was when weekly spelling and math fact tests started each Friday. I had to stop saying cheerfully on Thursday evening, "tomorrow is Friday!" something I thought would put most kids in a good mood, but it just stressed her out thinking of the timed tests the next day. She is also the kind of kid who would LOVE to stay home all day watching TV and playing computer games. A few times I have let her stay home only to realize I made the wrong call, she should have gone to school. Now I have some rules that work great with both my kids. If you are too sick to go to school in the morning, you go back to bed. I allow no TV or screen time if there are no obvious symptoms of illness like vomiting, high fever, etc. They also have to lay down in their bed after lunch and take a nap or rest. I have the school send their homework with to us with a neighbor kid, which I have them work on that evening. There are no evening activities, playing outside with neighbor friends, or going out of the house at all if they stay home from school sick. So I am a mean mom, because I make it incredibly boring to stay home sick if they only have a minor discomfort and not a real illness. And my kids hate to be bored, they'd much rather go to school, so it works out really well. If they really are sick, they actually want to be in bed and don't even care about TV.
I also don't make any promises the night before someone is complaining that they can stay home from school tomorrow. If they are asking to stay home the next day, I always just say, "we'll wait and decide on that in the morning." Many times they will feel better after a night's rest but if you've already put them in the mindset they'll *may* be staying home, they'll be wanting to go down the staying at home path the next day.
Unless one of my kids was sick the day before, I make them get up, eat breakfast and get ready for school each day, even if they're on the fence, they start getting in motion. I watch how they eat breakfast. If they eat normally, I take that as a good sign they'll be fine at school. Not eating at all, I'm more concerned it may be a real issue because they are normally good breakfast eaters. Often after they get out of the groggy stage, that little bit of phlegm in their throat doesn't bother them so much and they're fine. So I don't talk about staying home sick too soon. I act as if everything is normal and she is going to school and chances are she will fall into her regular routine unless she is really sick. If she is dressed and ready and still complaining, yet absent of physical symptoms, I ask they go and just try to be at school for the day. If they are feeling too poorly to concentrate or participate at school, they can call me. Almost always they stay at school. Once they are there, they very often get distracted from their little irritations.
If one of my kids does call me from school that they feel sick and want to come home, I will go and pick them up right away. But they know when we get home, they go take a nap. I'll ask them if they'd rather be home napping or at school if I think they are on the fence. If they really are sick, they really really want to be in bed. Otherwise, they may choose to stick it out for the day.