Calculating Whole School Total Tuition

I am trying to calculate a whole school total tuition. I've got helped with individual tuition total which is

create proc sp_individualtuition
as
select c.contractNum, (c.tuition-((sum(d.discountPer))* c.tuition)) as totaltuition
from contract c, contractDiscount cd, discount d
where c.contractNum = cd.contractNum
and cd.discountNum =d.discountNum
group by c.contractNum, c.tuition

however how can i calculate the whole school total tuition (adding all the individual total tuition)?

my contract, contractDiscount and discount DDL are down below

create table contract(
contractNum int identity(1,1) primary key,
contractDate smalldatetime not null,
tuition money not null,
studentId char(4) not null foreign key references student (studentId),
contactId int not null foreign key references contact (contactId)
);

create table contractDiscount(
contractNum int not null,
discountNum char(3) not null
);

alter table contractDiscount
add constraint pk_contractdiscount primary key clustered (contractNum, discountNum)
;

alter table contractDiscount
add constraint fk_contractdiscount_contractnum
foreign key (contractNum)
references contract(contractNum)
;

alter table contractDiscount
add constraint fk_contractdiscount_discountnum
foreign key (discountNum)
references discount(discountNum)

create table discount(
discountNum char(3) primary key,
discountDesc varchar(100) not null,
discountPer decimal(3,2) not null
);

thanks
gazawaymy