There are two main ideas about passwords. Most everyone agrees on the big pieces. Avoid personal information. Use uppercase, lowercase, symbols, and numbers.
Google suggests replacing letters with characters (saying repl@ce instead of replace). I think that is a terrible idea. It just makes it harder to remember, and a computer will not have any problem making those substitutions. XKCD has a great explanation.
Having said that, it is important to talk about passwords throughout the year. Students will most likely be starting new accounts throughout the year, so a one and done solution will not help your students.
Five Minutes of Dig Cit:
Take five minutes and talk about why strong passwords are important. Be Internet Awesome has talking points. Walk through creating a password with your students. Talk to them out loud about what you would and would not pick