I can't find the "suitable" words that I can directly say to a person higher in position than me (like my English teacher). I want to be polite, but also say the truth :/
I would suggest "not engaging." It seems to me to take some of the weight off of the teacher by implicitly acknowledging the audience as part of the interaction. A perfectly good lecture may fail to engage and earn constructive criticism without implying a substantive shortcoming in the teacher.
(Of course, offense is often in the delivery and that part is up to you.)
Dry would be a pretty good choice as it is not a purely negative way of describing something that while informative, is not especially attractive in style. From Merriam-Webster:
12 a: not showing or communicating warmth, enthusiasm, or tender feeling : SEVERE
a dry style of painting
b: WEARISOME, UNINTERESTING
dry passages of description
a dry lecturer
c: lacking embellishment : PLAIN
14: marked by matter-of-fact, ironic, or terse manner of expression
a dry wit
has a very dry sense of humor
May I suggest:
- vanilla - Referring to the flavour, sort of means "standard". Nothing exciting about it, but not necessarily bad. It gets the job done.
- mundane - Lacking excitement. Basically means "not interesting". Could be considered insulting still.
- pedestrian - Same as above.
- unvaried - This one is probably not going to be insulting. It just means that the teacher wasn't very dynamic. A bit monotonous, but they got the job done.
It may also help to provide some more context. Why do you want to say they are boring or not interesting?