Update Rows With Next Higher Id, If It Exists

I am trying to determine the next registered session of a student so I
can calculate the number of skipped sessions.

Scenario: I have a student registration summary table. One row for
each student and the student's registered session. I want to update a
given row with the next higher registered session (into a field called
next_registered_session_skey if the row exists). I can then use the
diff of the skeys to determine how many sessions the student skipped
for each registration period.

Example: Student X registers each fall for one session for 4 years.
The file might look like:

STUDENT_ID SESSION_ID SESSION_SKEY
NEXT_REGISTRED_SESSION_SKEY
123456789 200201 100
null
123456789 200301 104
null
123456789 200401 108
null
123456789 200501 112
null

I need to update the NEXT_REGISTRED_SESSION_SKEY so I end up with:
STUDENT_ID SESSION_ID SESSION_SKEY
NEXT_REGISTRED_SESSION_SKEY
123456789 200201 100
104
123456789 200301 104
108
123456789 200401 108
112
123456789 200501 112
null

I can then say SESSIONS_SKIPPED = NEXT_REGISTRED_SESSION_SKEY –
SESSION_KEY (logically speaking, not syntactically)

This is what I have so far as example:
UPDATE F_REGISTRATION
SET NEXT_REGISTERED_SESSION_SKEY = (
select top 1 nextr.session_skey
from f_registration r
inner join
f_registration nextr
on r.student_skey = nextr.student_skey and nextr.session_skey[color=blue]
> r.session_skey[/color]
order by r.session_skey desc
)
WHERE STUDENT_ID = '577665705';

SELECT student_skey, student_id, session_id, session_skey,
next_registered_session_skey, * FROM F_REGISTRATION WHERE STUDENT_ID
= '577665705' order by session_skey desc

RESULTS:

STUDENT_SKEY STUDENT_ID SESSION_ID SESSION_SKEY
NEXT_REGISTERED_SESSION_SKEY
125137 577665705 200404 309 311
125137 577665705 200403 308 311
125137 577665705 200402 307 311
125137 577665705 199804 285 311
125137 577665705 199803 284 311
125137 577665705 199802 283 311
125137 577665705 199704 281 311

TIA

Rob

(I restricted with the where = ‘577665705' so I did not have to wait
to update all the rows)