For bromine within it’s electron configuration the valence electrons are 4s2 and 4p5 and not 3d10 and 4p5, my chemistry teacher explained this as 4s2 being less shielded than the 3d orbital.
Valence electrons are the electrons that get ionised correct? If the 4s2 orbital experiences more Zeff then wouldn’t it be harder to ionise as there’s a greater degree of electrostatic attraction? And the 3d orbital, since it experiences more repulsive forces, would be easier to ionise?
Most likely your teacher misspoke. The $4s$ electrons clearly suffer less nuclear charge and are more reactive; in fact $4s$ and $4p$ the only reactive electrons in bromine.
In this answer the change in electronic energy levels as we add more nuclear charge is discussed: it causes the $3d$ orbitals to ultimately fall below the $4s$ in energy; $3d$ meets up with its $3s$ and $3p$ shell-mates as the electrons sort out according to their proncipal shells. With chromium and iron, the elements discussed in that answer, we need to form a positive ion to make this switch happen; but by the time we get to bromine there is enough unshielded nuclear charge even in the neutral atom to sink the $3d$ level below tge $4s$. Thus in bromine the $3d$ electrons are sunk into the core along with tge rest of the $n=3$ shell, leaving only the $4s$ and $40$ electrons to act in ordinary chemical processes.
