The innocent camp have to ask themselves why a trainee teacher would jeopardise everything just to exact revenge. Yet, as explained before, people do behave in irrational ways.
I believe these are the possibilities:
(i). Julie is telling the truth and Jeremy is guilty.
(ii). Julie is telling the truth but Jeremy is innocent.
(iii). Julie is lying because she was Jeremy's 'constructive accomplice'.
(iv). Julie is lying and Jeremy is innocent.
(v). Julie is lying but Jeremy is guilty anyway.