Other forms: favoritisms
A teacher who shows a clear preference for a particular student might be accused of favoritism, or favoring one person over others.
The noun favoritism can simply describe the habit of preferring one particular person or group of people, but it often implies that it's done at the expense of other people. Your grandmother practices favoritism if she clearly likes one of her grandchildren best, and a politician is guilty of favoritism if she gives tax breaks to one group but not to others. The word favorite is at the root of favoritism, from the Latin root favere, "to show kindness to."