What's a more polite word for 'boring' or 'not interesting'?

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