Capitalize it when it's part of a formal title:
* Bilingual Third Grade (This could be the name of a program, a class, or a specific group of students)
Don't capitalize it when it's used descriptively:
* The students are in a bilingual third-grade class.
* They are bilingual third graders.
Other Considerations:
* Check the style guide: If you're writing for a publication or organization, their style guide might have specific rules for capitalization.
* Context is key: Pay attention to how you are using the phrase to determine whether it needs to be capitalized.
Let me know if you have a specific example and I can help you decide!