So rediculous. Tell the dean you are not an actor and ask him/her to bring some students or you can post an advertisement to invite students yourself.

Try and get them involved as usual.  If the audience is just the DOS then tell him/her to arrange the chairs in a circle and find a partner .

Tell them white monkey is tired . That is what i always tell my Dos when he asks me to do ridiculous things . He finds it amusing . 

Practice pretending just get a doll and act like they are the student. 

Borrow some of Sparkeys dolls. Or make up imaginary students, you can have anyone you want: Kate Beckinsale, Keira Knightly, Megan Fox, Natalie Portman, Jessica Alba, Rachel McAdams, Olivia Wilde. You may want to keep some of these after class.

is sth wrong with me? where is the difficulty? you are a teacher? you teacher lessons for a goddam living! and you are having trouble thinking up a 15 minute, you can do whatever the puck you want lesson? seriously? i once did a whole hour on the spot with 5 minutes notice. i do not even prep for demos. I do my first lesson where i explain the basics of speaking english. I guess because i only take jobs dealing with students that are in my wheel house. if you can't teach the age group don't take the pucking job. 

It is a ridiculous idea, but not unheard of. If you are an experienced teacher, it would be quite simple, but annoying.

I agree with crimo. If you can't do a 15 minute demo class, you're in the wrong job. With online teaching now, teachers often have to record lectures with no students, so it is really no different.

Well I've done this same thing like say thrice or more. These are what i can suggest to you 

1. There will be no students of course, meaning you have to assume your soon BOSS-to-be and the other teachers who might be present as the students in all course.

2. Remember that your assumed students are not of the same age as the real students so what a student of 12/13/14/15 years will be excited and laugh his/her head off , an assumed student of say 30-45 years will not find it excited 

3. So from point 2, do treat your assumed students as they really are and do what you will to make the real students laugh to them. 

4. Involve them in the class, ask them questions and give them a little time seconds to brainstorm and give you the answer. if anyone ever answers you correctly, don't hesitate and give them the hi-5 or what i call the BIG O( both of you make a fist, then you lightly hit at the top and bottom of each other's fist in opposite direction then you take a head-on collision and finally you both raise up your two hands and clap in the air) 

5. The key thing is that you have to get them excited. You can also decide to give a short crazy dance for someone( assumed students) who does well at the end of the class. 

6. You shouldn't be too serious, if not you will fumble a lot and mess up things 

7. Don't talk too much, don't use many sentences or phrases. Go for the actions and signs 

Hope these will be of help. Gear up man, you can do it. Jia you jia you jia you