Dear @Geovany Palacios,
Thank you for posting your question in the Microsoft Q&A forum.
May I ask to clarify, is your issue that the 'Join' button for your class doesn't appear on the Teams app on your iPad, even though the class is currently in progress? Please confirm if I have understood this correctly.
Also, was this meeting created by your professor within a specific channel? If possible, could you please provide a screenshot of your current interface?
Since I don’t have an iPad environment available for testing right now, I can't check it directly, so a screenshot would be very helpful. However, on my iPhone, I can see the Join button by following these steps: Go to the Team > Select the Channel > Tap on the meeting post > Switch to the 'Details' tab.
Please let me know more about your situation so I can better assist you.