How to measure the amount of calcium in a supplement?
For the determination of calcium (at least) two methods are conceivable:


gravimetric analysis
Sulfate is a rather bad choice here. As compared to the carbonate, the sulfate has a much higher solubility. A better choice would be the oxalate with a solubility around $6\,\mathrm{mg\,l^{-1}}$.
photometry
Calcium forms a purple complex with the Arsenazo III dye that can be detected at wavelengths of 660 and 700 nm.
Alternatively, you can use o-cresolphthalein and detect the complex at 545 nm. 
The photometric methods are typically used in automated high-throughput clinical analysis of calcium in serum, but given that a photometer isn't available in the students' lab, Karl's comment on EDTA is exactly what you might be looking for. Complexometric titration is the keyword you might want to look for. Here, an indicator that forms a coloured complex with $\ce{Ca}$ is replaced by EDTA. The liberated indicator then shows a different colour.