This is not easy.
The following are questions only about user input when they overwrite formulae:
1. Can you confirm that the codes you mention are the ONLY codes a user will input:
XTI, XTIX, XLE, XLEX, LTI, LLTI, CLE, CT along with LLE, LLLE, C, CTI which you mention elsewhere
plus:
'X' absent am
'XX' absent all day
'XL' absent am, late pm
'L' late am
'LL' late am and late pm
'LX' late am, absent pm
plus:
(I proposee that anything after a space or spaces pertains to the afternoon, even when that space is the first character)
(It's not PHP code; it was a way to prevent this site removing multiple spaces from my text.)



I notice that some single X characters and some single L characters in your sample sheet are right aligned by cell formatting to signify pm. This is not reliable (and you'll have people trying to right align one character, and left align another within the same cell!). I'd recommend all cells to be centre aligned and people use a space or spaces to distinguish am from pm.

This leaves a few questons:
2. C for cutting classes (what does this mean?) does not have an attachment to am/pm?
3. Are other combinations possible? Eg.:
TIC
LEC
LETI
TILE
etc.etc?