A pupil is late if they arrive after the start of school but before the register closes (in most schools up to 30 minutes after the start of school) If they arrive after this see below for advice on which code to use.
Code L: Late arrival before the register has closed
Schools should have a policy on how long registers should be kept open; this should be for a reasonable length of time but note that registers are to be kept open for the whole session. A pupil arriving after the register has closed should be marked absent with code U, or with another absence code if that is more appropriate. (School attendance - Departmental advice for maintained schools, academies, independent schools and local authorities November 2013)