新手请问scanf()自动跳过的有关问题

#include "stdio.h"
#include "stdlib.h"
#include "math.h"

double teacher_total(double _amount)
{
    double total;
    printf("Total purchases                                  $%6.2lf\n", _amount);
    if(_amount >= 100)
    {
        total = _amount * (1 - 0.12);
        printf("Teacher's discount (12%%)                            %6.2lf\n", _amount * 0.12);
        printf("Discounted total                                  $%6.2lf\n", total);
    }
    else{
        total = _amount * (1 - 0.10);
        printf("Teacher's discount (10%%)                            %6.2lf\n", _amount * 0.10);
        printf("Discounted total                                  $%6.2lf\n", total);
    }
    printf("Sales tax (5%%)                                       %6.2lf\n", total * 0.05);
    printf("Total                                             $%6.2lf\n", total * 1.05);
}
double non_teacher(double _amount)
{
    printf("Total purchases                                   $%6.2lf\n", _amount);
    printf("Sales tax (5%%)                                       %6.2lf\n", _amount * 0.05);
    printf("Total                                             $%6.2lf\n", _amount * 1.05);
}
int main()
{
    double amount;
    char is_teacher;
    printf("KEITH'S SHEET MUSIC CALCULATOR\n");
    printf("Please enter the total amount for purchase ($):\n");
    scanf("%lf", &amount);
    printf("Are you a music teacher? ");
    scanf("%c", &is_teacher);
    printf("\n");
    printf("RECEIPT\n");
    if(is_teacher == 'Y')
    {
        teacher_total(amount);
    }
    else{
        non_teacher(amount);
    }
    system("PAUSE");
    return 0;
    
}