How to measure chlorine content using ORP sensor?
For a school project like yours, you can use iodometric titration. Iodometric titration is one of standard methods of water examination. The amount of free chlorine in tap water is really small on the order of &lt; 1 parts per million therefore you need a lot of sample. One would typically take 500 mL of tap water (or more), add potassium iodide, and titrate with 0.01 M sodium thiosulfate. You can use an ORP and plot a potentiometric titration curve.
Consult your teacher for this option. He/she should explain to you the details.
There are spectrophotometric methods but the reagents may not be available in your school.