I enjoy reading this manga, but this chapter was definitely insane. Apparently she had two orgasms (1. When she flat-out masturbated (page 16) and 2. When he spanked her (page 20)) in the course of doing her makeup exam... Even the most innocent of 10yo girls is going to realize something unusual happened, even mild orgasms are kind of intense.
The second orgasm might be arguable, but her panties have a very noticeable (and large) wet spot on page 19 from the first one.