ABC/WindowAdmin.xaml

<Window x:Class="School.WindowAdmin"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:School"
        mc:Ignorable="d"
        d:MinHeight="140" d:MinWidth="220"
        MaxHeight="140" MaxWidth="220"
        Title="Код администратора" Icon="/image/school_logo.ico" >
    <Grid>
        <StackPanel Background="#E7FABF">
            <GroupBox Header="Введите код">
                <TextBox Name="admincode"></TextBox>
            </GroupBox>

            <Button Name="Add" Template="{StaticResource ButtonInElement}" Click="Add_Click">Сохранить</Button>
        </StackPanel>
    </Grid>
</Window>