There are loads of things wrong with that code...
dateTimePicker1.Value.ToString() = prop.AdmissionYear;
Strings are immutable, so quite what you expect to happen when you try to assign a string object a new value I don;t know. You can assign a value to a string variable, but not to an object.
Dates are not just a year: they have months and days as well. So you probably want to use the year to generate a "proper" date which would be around the start of the academic year:
dateTimePicker1.Value = new DateTime(int.Parse(prop.AdmissionYear), 9, 1);