Removing Substring in asp.net with c#

I think the best solution is use string .split() method and select the first array

string[] str=text.split(';')

str[0]=Angel Nursery School;

You are using Substring method wrongly. Here we go:

Substring has two overloads :
Overload 1: Substring(int Starteindex) - It will take the start index and will return all the characters after the start index
e.g.

class Program
{
    static void Main()
    {
	string input = "Angel Nursery School; 001";
	string sub = input.Substring(6);
	Console.WriteLine("Substring: {0}", sub);
    }
}

It will return : Nursery School; 001

Overload 2: Substring(int Starteindex, int Endindex) - It will return all the characters between startindex and Endindex
e.g.

class Program
{
    static void Main()
    {
	string input = "Angel Nursery School; 001";
	string sub = input.Substring(6, 13);
	Console.WriteLine("Substring: {0}", sub);
    }
}

It will return : Nursery

Now coming back to your case. If you want to remove 001, then modify your code as :

String text = ddlschoolname.Text;
String numbers = text;
if (text.Length >= 10)
{
    numbers = text.Substring(0, text.Length - 3);
}

Hope this helps.
All the best.