You can choose to either donate or sell your old school uniform. Donating would be commendable as it means that you would be giving your uniform directly to someone who needs it. For example, perhaps a family member or a friend intends to attend Concord High School and needs a uniform. Instead of having to purchase a uniform, they would already have one thanks to you. In addition to the above post, if no one close to you needs it, you can simply donate it to an organisation such as the Salvation Army. That way, you would be making a contribution to the community.
On the other hand, you could sell your old school uniform. You can consider doing so if there is no family member or friend who needs it and/or if you do not wish to donate it to charity. Additionally, if you think that you would be better off getting some money, then that could be also a reason to sell your uniform. Of course, that depends on your circumstances. In terms of places where you can sell your uniform, you can try
The Uniform Exchange, which is a uniform store that buys and sells second-hand uniforms.
I hope this helps!