Using SQLite in Android to get RowID

I am creating an app to record assignments for different classes. Each class has it's own unique ID so the assignments listed for each class don't overlap into other classes. Here is a method I made to find the rowid for a certain class.

public int getIdFromClassName(String className){
    String query = "SELECT rowid FROM " + CLASSES_TABLE_NAME + " WHERE " + CLASSES_COLUMN_NAME + " = '" + className + "'";
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor res = db.rawQuery(query, null);
    return res.getColumnIndex("id");
}

However, this always returns a value of -1.

Any thoughts on what to change to return the proper rowid value?