As I recall, kids and teachers would regularly be low level sick during school, which seems to be supported here:
“It varies a lot among kids. But most babies, toddlers and preschoolers can have as many as 12 colds a year and still be normal. On average, you’ll probably see between seven and eight infections a year. This number may be higher for children who are in child care or when they start school. It’s also typical for kids to have symptoms lasting up to 14 days. And sometimes a cough can last up to six weeks. That means kids can be sick for a majority of the year and still have a pretty typical immune system.” https://mcpress.mayoclinic.org/parenting/the-constant-cold-w...
Kids are basically building up their immune systems. With poor hygiene practices and a tendency to touch everything (and then their face or mouth) it's unavoidable.
That being said, I do remember that the first cold season after people started going out again being nastier than normal.
“It varies a lot among kids. But most babies, toddlers and preschoolers can have as many as 12 colds a year and still be normal. On average, you’ll probably see between seven and eight infections a year. This number may be higher for children who are in child care or when they start school. It’s also typical for kids to have symptoms lasting up to 14 days. And sometimes a cough can last up to six weeks. That means kids can be sick for a majority of the year and still have a pretty typical immune system.” https://mcpress.mayoclinic.org/parenting/the-constant-cold-w...