Something like the following?

WITH absences AS (

SELECT *, COUNT(absence_dt) OVER( PARTITION BY student_id ORDER BY attendence_dt ROWS UNBOUNDED PRECEDING) AS absence_cnt

FROM attendence

WHERE attendence_dt >= DATEADD(DAY, -30, @begin_dt)

)

SELECT *

FROM absences

WHERE absence_cnt = 7

Drew

Edit: Added ROWS UNBOUNDED PRECEDING