To reproduce:
Add an appointment with the text "<html><size=9>Erin Swardz</br><color=Red>PO 2315</html>" the appointment looks formatted in the scheduler, however when in PrintPreview/Print the html code is printed in raw format
Workaround:
Strip all html in order to print pure text - 
void scheduler_AppointmentPrintElementFormatting(object sender, PrintAppointmentEventArgs e)
{
    string replaceBr = e.AppointmentElement.Text.Replace("</br>", " ");
    string result = Regex.Replace(replaceBr, @"<[^>]*>", string.Empty);
    e.AppointmentElement.Text = result;
}