Holy Batman people!! I only read the first page of replies and have a different point of view as some of the others.
OP - your son is in SECOND GRADE! He is a little guy, who happens to have issues with impulsivity. Mine does too, but it comes in the form of calling out during class. they have ADHD for god's sake! Your DS is not a violent kid, he's a sweet boy that didnt realize the harm that a dull scalpel could do. Use this as a teaching moment for sure. Those goddamn morons at your school should know better than to pose that line of questioning to your son by the way. WTF?! My DS, who is in 4th, would not know how to answer that question without getting himself into trouble either! he's a super honest kid who does not know how to lie or manipulate. What a bunch of beurocrats
Punishing him is over the top too. don't take away his screen time, or send him to his room. he's being punished enough for being reprimanded at school and brought home for the day. Then when he goes back he's going to be watched like a hawk and don't think he wont notice it. Have an open discussion as a family about what happened, and let it go. check his pockets and backpack in the morning going forward. You dont need counseling, you're not raising an evil kid. He has impulse issues not evil thoughts!