Irule to External Data Group not pulling up second data field

Your code looks reasonable. Are you just not returning a value into $goto? The class lookup command uses an implied "equals" comparison, so you might just not be making an exact match. Try this instead:

set goto [class match -value $match_string starts_with www_ou_edu_redir_class]

where the data group is the least specific URL pattern. You can also simplify some of your code by getting rid of a few variables:

set goto [class match -value [string tolower [HTTP::host][HTTP::path]] starts_with www_ou_edu_redir_class]