1/3 of one table = 1 student. 1/3 of two tables = 2 students. 1/3 of one table + 1/3 of one table = 2/3 of one table. 2/3 of one table = 1/3 of two tables.

1/3 of one table = 1 student. 1/3 of two tables = 2 students. 1/3 of one table + 1/3 of one table = 2/3 of one table. 2/3 of one table = 1/3 of two tables. 1/3 of one table + 1/3 of one table = 1/3 of two tables. -- It would take a lot longer to explain than to write, but that's how I'd be tempted to proceed. As far as learning the basics, it sounds like this class was just getting them introduced to the concept of fractions, while the arithmetic rules would come later...