I personally do believe that Sensei had tutored Maya before due to the same reasoning as yours, so the easiest way to think about Ami’s reaction is like what ppl mentioned: a mixture of jealousy and denial (she is a yandere after all). Unfortunately, it's difficult to deduce anything since we currently still don’t have a complete picture of the past; the information is always some descriptions one-sided from Ami, Maya, or Noriko (at best, some happy scenes or hectic events like Maya’s nightmare during the latest reset might offer some hints, but they're just confusing by design). It’s not entirely impossible that some of the things any of them said is partial truth, their own version of truth, their own vision of what should’ve been the truth, or just flat out not the truth (Perception matters!). Remember the time Sensei believed that Ami and Ayane kissed before during older times only because Ayane said so? Let's take things these girls claimed with a grain of salt before discerning who's lying or manipulating.
However, I do feel like there is always a chance of Sensei had tutored Maya before and You never tutored Maya before both being the truth, given how “happy” this event seems. More importantly, we have already seen how this world of reset can rewrite a certain person's certain memory while leaving others' memories as-is (ex1: No one remembered Makoto's change after Blue jay except for Maki; ex2: pregnant Ayane's memory right before reset being rewritten to not knowing the sleepover existed, while Sana still remembered and actually attended). Could Ami's perception insertloudbang regarding the fact that Maya once being tutored by Sensei was rewritten unbeknownst to everyone? Could it be the other way around?
In addition, since this event is "quite happy", it forces me to wonder if there is some influence from celestial beings involved. Ami is the daughter of Sekai and Nozomu (or potentially Sensei), and has quite the history of "freaking out". If this indicates that Ami can also be "touched by god" or "logged in", perhaps the statement You never tutored Maya before could be from the aspect of someone other the Ami (or still Ami but under some sort of influence) referencing a version of Maya we never met (imagine Sensei at the time is in fact talking to HOPE or Sekai the whole time). But of course, the fact that literally no one knows about where Maya came from makes it impossible to solve the puzzle. Thus, for now, we can only speculate before more vivid hints are disclosed. All we can agree on for now is that:
- The something bad Ami suggested right now is mostly believed to be hinting Sensei impregnating Maya or least being intimate to Maya
- Ami is creepy af (this girl scared the shit out of me more times than I am willing to admit)