17 - It's good for banging your head against when frustrated. It makes a big noise and doesn't hurt so much.
18 - A place for students to write homework answers. Then you can verify the answers are correct and let the students check each others work. Students will then have done all of the work - writing the answers and marking the homework
(In all seriousness, it's good for them to do this not just you the teacher all the time.)
(19 - my hands don't get chalky.)
(20 - my clothes don't get chalky.)
To the next person - the last 2 aren't really uses but votes for a whiteboard, so choose to count them if you wish.
You be the judge.