Well, the words "sneaky" and "silly" are not very nice. They would trip a defensive reaction in me, but seeing this in a mail to YOU it doesn't seem so bad :) EVERYONE'S kids act up sometimes. My oldest daughter is VERY well-disciplined and VERY well-behaved, but when she was in K4 she got some warnings similar to this, I explained to her it was not OK, I enforced the rules and checked in with teacher to make sure my daughter was behaving etc, it resolved itself.
Take the emotion away and think of it this way. The word "Sneaky" is kind of harsh, but he's obviously sneaking in some wrong behaviors. Just face it, embrace it, and address it. Don't feel hurt. If you WANT to let her know your feelings about how she's saying these things, do so, but you are a tad more hurt by what she said, than concerned for the behavior it seems. Let your ego go, no worries, just handle it.
If she would have said, "You know, it's normal for his age to push boundaries, but I do notice he tries to get away with some playful shenanigans (instead of silly) at times when no one's looking" you would have felt less offended perhaps....but whatever. You may not have realized there was a behavior problem either that way. Breathe deep, thick skin, she actually complimented your son overall.
It takes fortitude to swallow a less than stellar report, and accept it, and graciously handle it. But if more people did it, schools wouldn't be full of ill-behaved kids. :) Remember, you don't want your son to be hurt by firm direction from coaches, teachers, etc going forward, so model strong diplomacy for him now, not a "this hurt my feelings" outlook unless it's really warranted.