The captcha shown after too many incorrect attempts to log in looks ugly, compared to the nicely styled create account form captcha.

The signup captcha is being reformatted by a script called mediawiki.special.createAccount.js; it should be ran for the login form too (after being adjusted to work correctly).

See this bug report for more information. Students are required to read Wikimedia's general instructions first. You need to have the ConfirmEdit extension with FancyCaptcha enabled to see the captchas on your wiki.