T-SQL (SS2K8) :: Calculate Time In Minutes

I am having below schema:

CREATE TABLE #Attendance(
[ID] [int] IDENTITY(1,1) NOT NULL,
[StudentID] [int] NOT NULL,
[ClassID] [int] NOT NULL,
[DateAdded] [datetime] default getdate() NOT NULL
) ON [PRIMARY]

insert into #Attendance(StudentID,ClassID,DateAdded) values(1,1,'2014-10-07 10:38:02.900')

[Code] ....

DateAdded column in first table is nothing but in and out time.

Now I want to prepare a query where I want to consider MIN DateAdded and max DateAdded and calculate the duration of student present in the class.

Validations i need to consider are:

If class is starting at 10am then student can come at 9:50am, i.e. Dateadded column should consider as student present in that class if value is less that 10 minutes of StartTime from #ClassAttendance table. Class End time i want to calculate depending upon ClassMinutes from #ClassAttendance

Also DateAdded column should be 10 minutes plus compared to calculated endtime. If its more than that consider lower DateAdded time.

And by using this thingIi want to calculate total number of minutes student present in the class and number of minutes absent.

If there is only one DateAdded for class then consider as a absent student.