Negative oxidation state Why doesn’t aluminum have negative oxidation states? In almost all compounds of aluminum  which I know of has positive  oxidation state.

It has valence EC [Ne] 3s2 3p1 , my teacher told it usually forms +1 and +3 oxidation state by completely sharing p orbital electron or both s and p orbital completely(and +2 o.s. is always average of aluminum)

So why can’t aluminum or any 13th group element for that matter accept electron and make its p orbital half filled i.e form compounds with negative oxidation state?
Aluminum does assume negative oxidation states, see the list of elemental oxidation states in Wikipedia. Aluminum assumes a negative oxidation state when forming compounds with more electropositive elements like magnesium.

The magnesium compounds referred to above include $\ce{Mg_{17}Al_{12}}$.  In this compound there is enough magnesium to half-fill the valence $p$ orbital on aluminum and thus attain a $-2$ oxidation state for the aluminum, which is the minimum in Wikipedia's list. (Part of the magnesium would then be below $+2$).  However, this is only a formal designation. Magnesium aluminides are far from ionic in character.