One of the best things I've ever done for myself is spend 6 months working in retail (during the Christmas rush, no less). It was definitely outside my comfort zone, incredibly difficult for me, but it left me a far more confident person, able to make eye contact and communicate forthrightly with others in a way I was utterly incapable of prior to the experience. For much of the time, it wasn't fun, but goodness, it was great for me. So there definitely can be benefits to going outside of one's comfort zone. But that was on my own terms, I'm not sure if I'd want to push your son into something like that.
At the same time, I'd hope they're not talking about provoking an experience so extreme. It's possible for any child to stagnate if they're not pushed, the question is where you're pushing them, how far, and how hard. I'd ask for more information. Precisely what is the "comfort zone" they're talking about? Social? Or English, math, something else? And how do they intend to go about putting him outside of his comfort zone. In social situation, if they're expecting to put him outside of his comfort zone before he's ready and expecting him to sink or swim, he'll sink, and that benefits no one, least of all your son. Any such action really has to be very carefully thought out and monitored.
The idea could be beneficial if done right. But I'd probably ask for it to be expanded in the plan, or replaced by a different term to avoid misunderstandings, and that'll give you a chance to run through precisely what they're intending.
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"You're never more alone than when you're alone in a crowd"
-Captain Sheridan, Babylon 5
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