Howdy Heidi,
Thanks for comming to Moz! There are a few solutions for problems I'll try to give you mine for your problem.
You have two urls above:
http://www.mysite.com/classes/textures/?tab=register
http://www.mysite.com/classes/textures/?tab=homework
To me these look like two filters for one page (http://www.mysite.com/classes/textures/) so lets assume the content is the same for this page?
Now you have a canonical tag going here -https://www.mysite.com/courses/texture-classes"/>
This is a third url unrelated, this is fine if all these are duplicates and you want the canonical link to rank.
To make this all a little easier to under stand lets look at the canonical link:
its purpose is if you have duplicate content on pages (e.g. the same content but maybe its for different colours) you would use the canonical to point duplicate to one main page (say the original).
Now you can use this as an easier way to maange duplicate content especially for query base urls (a url with? in it like above).
Another option is to look at the pages (see 2 above) and ensure they are unique with content and meta tags etc.
There is a great tool called screaming frog which is very handy for helping find duplicate content as well as working out if its been fixed (you can recrawl as many times as you like) its free up to 500 urls
Lastly: if you fix something you have to wait as it can take a little while for it to appear as "fixed" in Google Webmaster tools (search console)
I hope that helps, and was what you were asking. I'm sure some other helpful Mozzers will also give some great ideas too!