Get Previous row record dynamically

Anonymous , I am assuming table two unique year month table.

Create a new column in both tables

Date = "01-" & [Month] & "-" & [year] // Change data type to date. 

Join both 1-M from Table 2 to table

Create a new column in table 2

Month Year sort = FORMAT([Date],"yyyymm")
Month Rank = RANKX(ALL('Table2'),'Table2'[Month Year Sort],,DESC,Dense)

Try measure like these example

This Month = CALCULATE(count('Table1'[Student- ID]), FILTER(ALL('Table2'),'Table2'[Month Rank]=max('Table2'[Month Rank])))
Last Month = CALCULATE(count('Table1'[Student- ID]), FILTER(ALL('Table2'),'Table2'[Month Rank]=max('Table2'[Month Rank])-1))