A heating pad can really help. Also, a Thermacare wrap is thin and can be worn under her clothes if she needs heat at school - we get the ones intended for the lower back and she wears it backwards, so the hot part is over her belly.
Also, an NSAID. Typically NSAIDs are not allowed for patients with Crohn's, but my daughter has severe juvenile idiopathic arthritis as well, so she is on one regularly.
But if it's just a couple days in the month and your daughter's IBD is controlled, her GI might allow an NSAID. We have found, through trial and error, that some NSAIDs are easier on the gut than others. Celebrex is one of them. My daughter was also able to tolerate Meloxicam and Nabumetone (all are easier on the stomach than Advil or Aleve, but all must be prescribed by a doc, as they're not available OTC). Additionally, if your daughter has stomach pain with them, she can take them with OTC Prilosec or Pepcid or a prescription antacid as needed to protect her stomach.
When my daughter was an older teen, she really struggled with severe cramps despite the NSAID and she also struggled with anemia/iron deficiency so was put on birth control, which made a HUGE difference both in pain and also made her period much lighter.