If it's been this severe every summer for three summers in a row - as someone else said below, you really need to get to the cause here. Just treating the symptom -- the tantrums -- with discipline is not, alone, going to uncover WHY she is so upset at the idea of school.
But there is far too much you don't tell us so we can't offer anything else to help. So much is missing. Here are some things you could think through here:
During the school year (not just in the weeks before school starts) does she also show a lot of anxiety? Does she hate to go to school all year long, or does she warm up to it and stop the tantrums and anxiety once she is in school and knows what to expect there?
Does she have anxiety about other things in life that create uncertainty, or ONLY about school?
Have you ever investigated whether she might be bullied there? (You cannot depend on her to just tell you -- kids can hide it very well; you need to have talked with teachers, other parents, the school counselor.)
Does she feel academically overwhelmed? Is it the actual schoolwork that she fears? Does she balk and fuss about homework all year long, or say constantly "I can't do it"? Or alternatively, does she seem bored and hate school because it's too easy (meaning she is very bright and is not being challenged enough)?
Have you ever had her see a school counselor or have you, alone without her, talked with the counselor about her behaviors?
Is it possible that the issue is not really school but she uses school panic as a way to get extra attention in the summer? That seems a bit unlikely to me --but it could happen, especially if she is fine and dandy once school actually starts. Do you give her heaps and heaps of attention when these I-hate-school tantrums begin? That could be an incentive for her to throw them every summer at about the time she starts to feel bored and ready for mom's attention on her. (But again -- it sounds as if that is not the case. Just something to consider if all else about school seems fine and these tantrums are confined only to a certain period in summer.)
Does she have enough to really occupy her in the summer? Is she mostly at home all day, every day, or just hanging out with siblings or friends, or is she busy in camps, classes, other stuff? She might need some organized distraction from her own worries, and if she's just at home and mostly expected to occupy herself, that could be what's giving her mind enough time and free rein to turn toward "I'm scared of school." I would ensure she was very busy outside home as well as at home. She needs something where an adult who is not you is in charge.
Does she cling to you strongly in other ways? She might need more of those outside activities without mom in order to build her confidence at being without you.
She has enough experience of school to KNOW by now that her tantrums will not get her the result she says she wants -- to be kept home and not sent to school. So the tantrums must be getting her something else she wants on another level. I suspect that something must be attention from you and/or a way to express her fears about the schoolwork, or bullying, or just the change of starting a new school year. But it sounds like you need to do some investigation to find out what is really the root of her fear.
Meanwhile I would be sure she was kept very busy; was doing a lot outside the house and in organized settings without you there; and that you don't talk up school too much. Telling her over and over that school will be great may just backfire here.