Exemption and privileges for public servants.

I am reading part conviction and part acquittals under section 34 of indian penal code.i have found out that if a b c d commit a crime in furtherance of common intention of all.out of these four a is not a public servant and left three are.a's guilt is proved beyond doubt but but sanction for the prosecution of b c and d is not granted by appropriate authority.here the conviction of a is good and all the three public servants will be acquitted. My question is can they be acquitted like this?are they any exemption or privilege to them?do the prior permission for their prosecution have to be taken from appropriate authority?if yes then what is that appropriate authority?can this type of acquittal be said to be in public interest?kindly help me