Hi Vivek,
This type of operation is going to require another table so that information can be summarized up and passed back down across records. Typically the way to think of moving or using data in Quick Base is that formulas can utilize values that already appear on a single record. So if you have two fields on the record you can use them both in a formula. If on the other hand a value you want is in another record you need to use a relationship most often to bridge that gap.
In this instance for example you could make a Class table that would hold your Class values. So it would be a table with Class 1, Class 2, Class 3 as records that you would then relate to each record you make on Table A. Then from there you could use a summary field to summarize up data to help you move data around across related records with some criteria built into the summary to get the exact values you are looking for. Once you have the right values summarized up you can use a look up field to move them back down to Table A, this set up is a more complicated way of moving data around.
Then from there you would have a field on the record you could work with for another formula or calculation in the record. This would require building out a formula field that could help you indicate if you want the value from the previous class or a manually entered value not dependent on the previous Class record for the most flexibility. I hope this information is helpful.
