OnePerfectOne has a very good idea: You --and the teacher and other aides or adults who see him daily -- need to look for patterns here, and triggers.
Do the same couple of kids always seem to be the ones around whom he acts out, and does he not seem to act out when he's not around them?
Does it start to happen more frequently later in the week? Later in the day? He might be tired--not just physically but tired of having to listen and do what he's told. Younger kids do reach a limit with that. They have to stay "switched on" to the "school setting" so to speak and eventually can't stay on any more.
Just before lunch (when he might be cranky and off-task because his blood sugar, and therefore his attention, is dropping)?
Just after lunch, when kids sometimes are restive because they've gotten through half the day of listening and obeying, and lunch has broken that a bit so they're having a hard time getting back into schoolwork in the afternoon?
At recess, where he's running around blowing off steam and playing hard with other kids, and is so into that that he can't set it aside when it's time to come in?
When he's asked to do certain specific school tasks that might be harder for him than the teachers, or you, realize? For instance, does he start to get off task when he has to write for more than a few moments? My friend's son who had issues with motor control was like that and they found it wasn't an attention issue; the physical act of writing was frustrating for him and he didn't want to do it. Maybe he gets off task when the kids have to sit and listen to a story? That's pretty typical. I've seen teachers move kids to new locations in the "story circle" seating or let them sit apart from the group if the kids were just more settled that way. Does his teacher pay that kind of attention and allow some flexibility for how kids listen, or does she insist everyone sit with perfect stillness in the same places?
As you can see, the gist here is: You need to forge a close working relationship with his teacher(s) and tell them you are on the watch for patterns and triggers. The behaviors you describe seem pretty typical for a child his age (first grade, right?). I am surprised that any experienced teacher would be contacting you three times a week-- is that three times based on some schedule you and she have set, or does it mean that he acts out severely enough, at least three times each week, that she is contacting you because of specific problems that day? If the latter -- most experienced teachers would cope better than to be in such frequent contact with parents, unless a child were truly disruptive and possibly hurting others. I am NOT saying "blame the teacher" but you should be working with her to find out the when and why of his acting up, not just getting reports about it.
Then you move on to reducing triggers: She keeps him pretty separate from kids who seem to egg him on; you arrange that he can have a snack before lunchtime if his attention tends to flag badly at that time most days; or she and you call in someone to evaluate him for motor skill issues if writing or other things seem to frustrate him unusually; and so on.
Yes, absolutely, he needs consistent discipline as well, at home and at school, and he needs to know that mom and dad always, always know what happens at school -- kids don't realize at first that their parents are aware they did something at school that day. Be sure he does understand that. However, what disicpline is used at school and how fast does it get used after he acts out? Discipline at home is important but he's still young enough that if the discipline comes hours after his action (iin other words, if he's only disciplined at home for something he did at school), the discipline has far less effectiveness. You and the teacher again have to work together on what she will do every time, consistently, and on how you will reinforce that at home. But it's not solely about discipline for "being bad." It's also about increasing the odds he can behave well by seeing if there are triggers that can be changed.
Bedtime tantrums are pretty normal but not if they're happening more than rarely. Look at your routines and his bedtime; is it too late? Too early, and he's not tired enough? Is he getting hungry again? Or is it all about his asserting himself as a separate person and you are not the boss of him? Do you let him choose the book you'll read to him (from a choice of two, don't overwhelm him by saying "Pick something off your bookcase" or he will dawdle for ages). Take apart the routine and see if you can improve it, slow down the process or give him a little control over some aspects of it. If he has siblings, it may be that he's trying to get your attention at bedtime because you're busy with younger kids and he still wants you.