You could simply nest the fields, as in:
{IF {MERGEFIELD percentage_attendance}>= 95 "*" {IF {MERGEFIELD percentage_attendance}>= 90 "**"}}
or:
{IF {MERGEFIELD percentage_attendance}< 90 "" {IF {MERGEFIELD percentage_attendance}< 95 "**" "*"}}

You could even forego the IF test entirely, with:
{=(INT({MERGEFIELD percentage_attendance}/5)-18) \# "'*';;'**'"}

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]