Schools take a lot of heat. For starters, they’re constantly criticized for teaching not enough of the right things and too much of the wrong things.

But new research shows that schools do a comparatively good job on one weighty matter: childhood obesity.

Conventional wisdom holds that schools are a major culprit in the expanding waistlines of our children. They’ve replaced nutritious meals with fast food, stocked vending machines with sugary drinks and banished physical education from the curriculum.

While there may be some truth to those claims, there’s more to the story. Recent research from Indiana and Ohio State universities suggests that schools may be better than parents at keeping kids fit.

Sociologists examined the body mass index of more than five-thousand children as they moved through kindergarten into first grade. While these children would normally be expected to grow, the researchers found that the period of maximum weight gain occurred during the summer months, when B-M-Is increased by about twice as much as during the school year.

Three subgroups were particularly at risk for putting on the extra pounds: blacks, Hispanics and children who were already considered overweight at the beginning of kindergarten. Once the kids were back in school, their growth rates fell.

The findings didn’t reveal the reason for this bulge in the data. The lack of a structured schedule during summertime could be partly to blame. So could the growing influence of video games and television.

And don’t forget that extra dose of grandma’s cooking.