Translation:Class is starting. Please sit down.
I think that you can say it. It seems to correspond to the use #4 here: https://www.perfect-english-grammar.com/present-simple-use.html
Although I'm not sure about how reliable that webpage is (I found it just searching for "use of present simple"). In any case, I have heard present simple used in that way in some occasions, so that use does exist, even if I don't know whether it is "officially" accepted