Google Script to see if text contains a value
Google Apps Script is javascript, you can use all the string methods...
var grade = itemResponse.getResponse();
if(grade.indexOf("9th")>-1){do something }
You can find doc on many sites, this one for example.
Update 2020:
You can now use Modern ECMAScript syntax thanks to V8 Runtime.
You can use includes():
var grade = itemResponse.getResponse();
if(grade.includes("9th")){do something}