SQL, JDBC "Invalid column index" error with simple query

I'm new to STRUTS and JDBC, my application tries to connect to a simple DB that has 3 tables, right now all is doing is trying to query 1 table that only stores "first, last names and a Id field"

System.out.println("-------- Oracle JDBC Connection Testing ------");

            try {
                Class.forName("oracle.jdbc.driver.OracleDriver");
            } catch (ClassNotFoundException e) {
                System.out.println("Where is your Oracle JDBC Driver?");
                e.printStackTrace();
                return null;
            }

            System.out.println("Oracle JDBC Driver Registered!");

            try {
                connection = 
                    DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:xe","david","changeit");
            } catch (SQLException e) {
                System.out.println("Connection Failed! Check output console");
                e.printStackTrace();
                return null;
            }

            if (connection != null) {
                System.out.println("You made it, take control your database now!");
            } else {
                System.out.println("Failed to make connection!");
            }

where I would like to get the result of 1 column if a match occurs:

            String sql = "SELECT S_ID FROM Students WHERE firstname=? AND lastname=?";

            PreparedStatement ps = connection.prepareStatement(sql);
            ps.setString(1, firstname);
            ps.setString(2, lastname);

            rs = ps.executeQuery();

            while (rs.next()) {
                studentid = rs.getString(1);
                ret = SUCCESS;
            }
        } catch (Exception e) { ...

As far as I can tell the connection is made, the SQL query

Select s_id from Students where firstname='first' and lastname='last';

when run on SQL Dev. works and gives me a single result.

I don't really get a stack trace the code just jumps from right before the 'while (rs.next()) {..' directly into the finally block

} catch (Exception e) {
        e.printStackTrace();
        ret = ERROR;
    } finally {
        if (connection != null) {
            try {
                connection.close();
            } catch (Exception e) {
            }
        }
    }