A private college or university is allowed to grant preferential treatment to anyone they please, except for oen of the reasons prohibited by relevant anti-discrimination law. Such laws vary by state, but generally prohibit preferences based on race, national origin, or gender. Some also include sexual preference or other categories. But I have never heard of a law preventing discrimination based on wealth, or specific donations. None of the Federal civil rights acts have such a provision.
Preferences for so-called "legacy" students -- that is students whose parent or parents (or possibly more remote ancestors) are alumni of the school are common, and I am not aware of any case in which they have been seriously challenged as unlawful discrimination. Preferences for military veterans are also common, and may be mandated for public schools.
For public schools justifying discrimination might be harder, and any relevant legislative polices will need to be considered, but even they can be free to make "rational" distinctions at least.
Not all discrimination is unlawful discrimination -- far from it.