Here is the answer for this question. I have done it for 20 students you can make it for any number of students by removing the condition i>=20 from the statement if((marks[i]>100)||(i>=20)). The java file is as follows

    import java.awt.Color;

    import java.awt.Container;

    import java.awt.FlowLayout;

    import java.awt.event.*;

    import java.awt.event.ActionEvent;

    import java.awt.event.ActionListener;

    import java.awt.*;

    import javax.swing.*;

    import javax.swing.BorderFactory;

    import javax.swing.JApplet;

    import javax.swing.JLabel;

    import javax.swing.JButton;

    import javax.swing.JFrame;

    import javax.swing.JTextField;

    import javax.swing.border.Border;

    public class ClassStatistics1 extends JApplet implements ActionListener

    {

    float marks[]=new float[20];

    int i=0,ctr=0,ctr1=0,ctr2=0,ctr3=0,passed=0,num=0;

    float highest=0, lowest=101,sum=0,avg=0;

    JFrame f1=new JFrame();

    JLabel label1=new JLabel("Enter marks");

    JTextField tf1=new JTextField(10);

    private JButton b = new JButton("Add");

    public void init() 

    {

        Container cp = getContentPane();

    tf1.setEditable(true);

    b.setEnabled(true);

    cp.setLayout(new FlowLayout());

        // Create Borders for components:

        Border brd = BorderFactory.createMatteBorder(1, 1, 2, 2, Color.BLACK);

        tf1.setBorder(brd);

    cp.add(label1);

    cp.add(tf1);

    cp.add(b);

    b.addActionListener(this);


    }

    public void actionPerformed(ActionEvent e)

    {

    marks[i]=Float.parseFloat(tf1.getText());


    if(marks[i]>=0&&marks[i]<=29)

    ctr++;

    else if(marks[i]>=30&&marks[i]<=39)

    ctr1++;

    else if(marks[i]>=40&&marks[i]<=69)

    ctr2++;

    else if(marks[i]>=70&&marks[i]<=100)

    ctr3++;

    if((marks[i]>100)||(i>=20))

    {

    f1.getContentPane();

    f1.setSize(250, 375);

    JPanel p=new JPanel();

    p.setSize(250,375);

    p.setLayout(new GridBagLayout());

    GridBagConstraints c = new GridBagConstraints();

    int k=0,m=6;

    JLabel l1,l2,l3,l4,l5,l6,l7,l8,l9,l10,l11,l12;

    JLabel []lab=new JLabel[20];

    l1=new JLabel("0-29 ");

    c.fill=GridBagConstraints.HORIZONTAL;

    c.gridx=0;

    c.gridy=0;

    p.add(l1,c);

    while(ctr!=0)

    {

    m=m+3;

    lab[k]=new JLabel("");

    lab[k].setText("*");

    c.gridx=m;

    c.gridy=0;

    p.add(lab[k],c);

    k++;

    ctr--;

    }

    m=6;

    l2=new JLabel("30-39 ");

    c.gridx=0;

    c.gridy=5;

    p.add(l2,c);

    while(ctr1!=0)

    {

    m=m+3;

    lab[k]=new JLabel("*");

    c.gridx=m;

    c.gridy=5;

    p.add(lab[k],c);

    k++;

    ctr1--;

    }

    m=6;

    l3=new JLabel("40-69 ");

    c.gridx=0;

    c.gridy=10;

    p.add(l3,c);

    while(ctr2!=0)

    {

    m=m+3;

    lab[k]=new JLabel("*");

    c.gridx=m;

    c.gridy=10;

    p.add(lab[k],c);

    k++;

    ctr2--;

    }


    m=6;


    l4=new JLabel("70-100 ");

    c.gridx=0;

    c.gridy=15;

    p.add(l4,c);

    while(ctr3!=0)

    {

    m=m+3;

    lab[k]=new JLabel("*");

    c.gridx=m;

    c.gridy=15;

    p.add(lab[k],c);

    ctr3--;

    }


    avg=sum/num;

    l11=new JLabel("Average marks awarded: ");

    c.gridx=0;

    c.gridy=20;

    p.add(l11,c);

    l12=new JLabel("");

    l12.setText(Float.toString(avg));

    c.gridx=28;

    c.gridy=20;

    p.add(l12,c);


    l8=new JLabel("Passed: ");

    c.gridx=0;

    c.gridy=25;

    p.add(l8,c);

    l5=new JLabel("");

    l5.setText(Integer.toString(passed));

    c.gridx=28;

    c.gridy=25;

    p.add(l5,c);


    l9=new JLabel("Highest: ");

    c.gridx=0;

    c.gridy=30;

    p.add(l9,c);

    l6=new JLabel("");

    l6.setText(Float.toString(highest));

    c.gridx=28;

    c.gridy=30;

    p.add(l6,c);


    l10=new JLabel("Lowest: ");

    c.gridx=0;

    c.gridy=35;

    p.add(l10,c);


    l7=new JLabel("");

    l7.setText(Float.toString(lowest));

    c.gridx=28;

    c.gridy=35;

    p.add(l7,c);


    f1.add(p);

    f1.setVisible(true);

    }

    sum=sum+marks[i];

    if(marks[i]>=50)

    passed++;

    if(marks[i]>=highest)

    highest=marks[i];

    if(marks[i]<=lowest)

    lowest=marks[i];

    i++;

    tf1.setText("");

    num++;

    }
    }

    The html file is as follows

    <html>
    <applet code="ClassStatistics1.class" width="350" height="350">
    </applet> 
    </html>