The comedy is entirely centered around "Senpai" failing at the magic she is trying to perform, but it really warrants more of a reaction of pity than laughing. Comedy is subjective, but there are only certain people that can keep on laughing at the same punchline of "oopsies I failed at this magic trick, which got me in a pitiful situation" over and over again. The sad thing is that this could of been a compelling basis for the story, but then the story writer decided to make "Senpai" too arrogant and sadistic in her actions towards "Assistant" to feel any pity for.
As far as the ecchi scenes go, they are very well drawn, but terribly executed. All of the ecchi scenes are far out of left field and contribute basically nothing to the comedy, character development, or story. In a good show, "Assistant" would want to stay in the magic club to either help stop "Senpai" from exposing herself or because he enjoys it. Instead, all of these scenes go without any purpose or reason. In a good comedy, there would be some wacky occurrence that would lead to the fan service, but instead all of the fan service is due to the most mundane events. The ecchi scenes can also be almost impossibile to watch, as the story writer decided that it would be a good idea have "Senpai" have an ecchi scene (albeit relatively tame) in front of grade school kids. Seriously, who thought this was a good idea?
The best way to watch Tejina-senpai would be a compilation of all of the ecchi scenes, but at that point though, why would you? There are other shows that do ecchi scenes better. This show is a hard pass.
