Sorry to hear you are going through this especially at a young age. I went through it myself but was more when I was junior high/early HS age and there wasn't nearly the attention to it back in the 90's as there is today. Both my daughters (5th and 2nd grade now) have had some minor/isolated incidents at school that we've coached them through pretty successfully so far. The main thing we've had them do is if it's mostly something going on in the classroom then have your kid find a time during the day when they can approach the teacher without other kids around if possible, such as right before or after school or maybe be the last one out for recess and tell their teacher they need to stay behind to tell them something. That's worked pretty well for the situations we were dealing with because most of it was just 1 or 2 kids that were disturbing them during class and when the teacher has 20 or so kids to pay attention to at once they don't always notice it. After they talked with the teacher alone the teacher then paid more attention to what the kid(s) in question were doing and then were able to notice it and address the behavior and move them to a different part of the room away from them and curb the behavior. We tell both our girls that our last resort is for us to have to start communicating with the teacher about it if possible because once we do that it could possibly make the situation worse if the kids know we went to the teacher so the ideal thing is for them to appropriately make their teacher aware of what is going on while not doing it in a manner that looks like they are being a tattletale or crying wolf.
Had one of these small situations just last week with my oldest at soccer practice. Apparently 1 of the girls was spitting water at her when they were on the sideline and 1 time when my daughter was on the field and this girl was on the sideline she was shaking up her water bottle and attempted to put grass in it before another girl stepped in and told her not to then told my daughter when she came to the sideline what the girl tried to do (I am hoping to run into 1 of her parents sometime to let them know they should be proud of what she did!) My daughter said she tried to tell her coach about it but he didn't say anything but when he has a dozen or so kids to pay attention to while practice is going on he's probably not paying much attention to the sideline. Told her the easiest way to avoid the situation is when she is not on the field during a scrimmage or game to have her water bottle with her and stand closer to the coach instead of down the sideline where he is not near them as the girls tend to socialize or mess around when they are not on the field. No one is going to spit at or mess with her bottle if you are standing with coach and it also shows him she is engaged in practice too.
Hope you find resolution to your situation, every one is different so there is no perfect way to handle them. If something physical is happening document it, take photos if needed as that is the point I'd probably engage the school. Verbal bullying especially at a young age is a little more difficult to document because unless there are other witnesses willing to verify what is happening with your kid then it's basically a game of he/she said where 1 side is going to deny it. Tactfully having your kid make their teacher aware of what is happening in a 1 on 1 setting so they hopefully will make a point to pay closer attention to what is going on around your child is my preferred way to start the process. Also coach your kid to remove themselves from situations when the bullying happens too. They don't necessarily have to retaliate but if it's recess just walk away and head in the direction of a teacher or someone supervising recess as the bully likely is not going to follow and try something in sight where they would be caught.