try this
if your Drop Down list contains only the Schoolcode as the display member,simply use the code,
string qry = "Insert into SchoolBranch (BranchCode,City,State,Country,SchoolCode) values ('"+Sbranchcode.Text+"','"+SbranchCity.Text+"','"+Sbranhstate.Text+"','"+SbranchCountry.Text+"','"+BSchoolCode.Text+"')";
BusinessLogic.executqry(qry);
You can get the selected text value of a combo box by simply using the Text property of Combo box.
eg:
string s=cmbComboBox.Text;