Help me troubleshoot a problem?

DS is in 1st grade in Spanish Immersion. He reads many years ahead in English, and is a big reader, but this is about Spanish reading, which is what he's evaluated on in school.

K: (Dec) Spanish reading level = F (end of K)
K: (May) Spanish reading level = I (end of 1st)
1: (Sep) Spanish reading level = C (mid K)

He had a major regression over the summer, supposedly -- that's like a year and a half almost. He spent all summer in Spanish Immersion camp so I don't think it's a language regression.

I think the test went horribly wrong. The problem is now it's impacting his placement in class at school. My kid that's supposed to have an IEP for more challenging differentiation now isn't even in the high reading group, isn't getting the right level of reading books sent home.

I had him read an F level book and it was too easy for him, so I double checked the scores as best as I can. I know when you get into 1st grade assessments you have to have the "formula" they want for retelling the story, but that shouldn't happen until about an F level, so why did he test so poorly?

One thing he said to us in the first 2 weeks of school was that his teacher didn't seem to listen to him. Like she would ask him a question and then walk away as he answered it. Although he has 99% level scores, he has a low processing speed. I don't know if it's a real issue or not, because he's just a deliberate, intentional speaker. He will collect all of his thoughts and then speak.

Related to this the teacher said when she was testing him he didn't answer the questions, so she just assumed he didn't know the answer and move on.

I obviously need to push to have him retested but how do I avoid the same thing happening again if she's simply not giving him time to respond? Should I teach him to reply faster? I kind of see this as an okay part of who he is, not something that needs fixed, but am I being naive?

She also said he was having a hard time pronouncing 3 syllable words. He had no problem with me. It's almost like she tested a different kid, so strange.

Frustrating.