Modern TVs often have a setting hidden in their menus that allows people to be able to use headphones without the speakers being muted.
Older TVs use a more basic system, whereby inserting the headphone jack moves a little prong inside the socket, which breaks the circuit to the speakers. (With such a system, it's impossible to have both speakers and headphones in use at the same time with either getting inside the set or using external amplified speakers, as well as headphones, fed from the headphone socket).
So, in order to provide an accurate answer, we need to know the make and model of your set. (NB: We'll need the exact model number, from the back of the set. A general response, such as 'Sony Bravia' won't be sufficient).