a boy in school was staring at my chest, i asked him and he said he can see my nipples, i am wearing a tshirt bra and i am 14, how is my nipple visible

At your age, it's completely normal for your body to be developing, and certain fabrics or styles of clothing can sometimes be more revealing than others. A t-shirt bra is usually designed to be seamless and provide a smooth silhouette, but depending on the material of your shirt, lighting, and how fitted the clothing is, it's possible that the outline of your nipples could be visible. If it makes you uncomfortable, you might consider wearing a thicker fabric or layering with another shirt, or even wearing a different type of bra that provides more coverage. It's important to wear what makes you feel comfortable and confident! If you have any concerns about this or how to handle the situation, talking to a trusted adult can also be helpful.