Restric form send if entry was made for the same day

[felipegm]

My app is for managing classes.

I have a spreadsheet for student check-ins for a specific date and time slot (day or night).

However, if no student checks in for that day and time slot, the teacher will have to cancel the class for that day and time slot, preventing any further scheduling for that class by any student.

The submission of a class cancellation form by the teacher would create a new line on a Google sheet with the date and time slot of the canceled class, ensuring that no new students could schedule for that time. This record would later be used for scheduling students to verify if that class is still available.

How can I verify, upon submission, if the date and time slot the teacher will cancel is already booked on the sheet or not? I believe it would have to occur upon submission to prevent someone from submitting just before the teacher without their knowledge.

[jaichith]

As far as my understanding, 

You have 2 or 3 status for the date and time slot . 1. Cancelled, Available etc. 

This can be somehow done by using valid if condition. 

Teacher will make entries for day slot as cancelled or available . By default it will be available right ? 

You want to list down the dates and times to students if it is available right ?. 

You can make the valid if condition as select(tablename[dates], [status] <> "Cancelled")