Interesting that you make the statement, has always been unhappy with school, even in pre-k.

I have two very gifted, very intelligent daughters.
First daughter loved her first pre-school. Then we moved. The pre-school we sent her to in our new town was supposed to be the best and oh so experienced with above average kids. She was sad from the first day. I won't go into it, but I had a bad interaction with her teacher and my gut told me to pull her immediately. I re-enrolled her in her old pre-school and drove 40 minutes each day and she loved it. The next year I found a new pre-school in town which she also loved. K wasn't perfect, but she was reasonably happy.
She's had some sticking points in school, but luckily the administration has responded and tried to meet her needs and she is relatively happy now.
Youngest daughter had a great year in 3 year old pre-school. The next year was awful, she had a different teacher who didn't "get" her and was bored out of her mind. I pulled her mid year and placed her at our church pre-school. Again, miserable experience. Kindergarten year and she hates it. The teacher is not a good fit and certainly doesn't get her. The school is making an effort but I just am not sure it will ever be a good fit for her.
Thus, my location in purgatory. One is in heaven, one in, you know, the other place. That put's me uncomfortably in the middle.
There is not a whole lot of difference between them in terms of intelligence. It's their personalities that are so different.
We will try with the school for next year, but I am prepared to homeschool if it doesn't work out. Only the child suffers in a situation where the school environment is not a good fit.