It is not uncommon for adhd kids to get frustrated (especially if they are intelligent) and lash out. Many times this is followed by anxiety and depression if left untreated. Losing all self control is kind of scary, but could be due to several factors. By the way this is something that can be dealt with.
Does he have this problem of the violent outbursts only at school? If so this really needs to be looked into because there is something going on at school that could be provoking this.
Do the outbursts happen only after certain subjects? or certain times of the day? Many times lack of sleep can cause kids to really lose it later on in the day (and this can be dealt with)
I assume he is not on medication yet? Typically medication can help with the frustration because he will be better able to focus. If he is on medication...then it may be too low of a dose and losing effectiveness or could even be too high of a dose. There are ways to tell which one is going on.
When is his birthday? is he the youngest in his class? This can lead to frustration and many times will cause teacher expectations to not match his ability to produce.
Finally no matter what the reason he needs to learn how to gain self control. Part of that is his teachers noticing when he is getting close to losing it and intervening. But more importantly there are things that you can do at home to help him learn how to deal with his anger. There are some great books aimed at this age group. They are meant to be read aloud and then practiced (many times). I will give you the links to two really good books and on that site you will find other good suggestions.
The first is "When I feel angry" https://www.amazon.com/When-Feel-Angry-Way-Books/dp/0807588970/ref=pd_sim_14_1?_encoding=UTF8&pd_rd_i=0807588970&pd_rd_r=f729ae54-2b2c-11e9-876d-3113f2f58f1c&pd_rd_w=gM2u3&pd_rd_wg=91Xun&pf_rd_p=90485860-83e9-4fd9-b838-b28a9b7fda30&pf_rd_r=PYM18C18XY01CCBHX8XY&refRID=PYM18C18XY01CCBHX8XY&th=1
The second is "Cool down and work through anger" https://www.amazon.com/Cool-Through-Anger-Learning-Along%C2%AE/dp/1575423464/ref=pd_sim_14_7?_encoding=UTF8&pd_rd_i=1575423464&pd_rd_r=0c1b509c-2b2d-11e9-b5a3-830356680a3c&pd_rd_w=uUBat&pd_rd_wg=Pl4WN&pf_rd_p=90485860-83e9-4fd9-b838-b28a9b7fda30&pf_rd_r=TGW2A4V5RX05NTHDFXTA&psc=1&refRID=TGW2A4V5RX05NTHDFXTA
and some other good ideas are here....https://www.additudemag.com/your-add-life-13/
I hope this helps. I will be glad to provide more resources as I get a better idea from you what is going on. Best wishes. !