Don't worry, 'it' (generalnie mowimy..everything will be fine) will be fine.
The Head Teacher didn't 'let' (mozna tez 'allow') you TO work at school or TO bring your (own) computer. So I have 'the' 'A' different idea. You should ask the Head Teacher 'to apportion' (tutaj lepiej...if he would agree to a) 'this' (niepotr) repayment 'among' (zle slowo tutaj potrzeba BY) instalments.
If he agrees, THEN, (I would advise you) as soon as possible look for 'the' (niepotr) work. You can work in 'the' A pub, 'the' A shop, you can
distribute leaflets.
You will earn money and you will 'giving back' RETURN (the sum owed) to the Head Teacher. (Prosze zauwazyc, Head Teacher piszemy duzymi literami)and
in this way you will pay for the damaged computer.
If you want to pay money back as soon as possible, you should pay your
'pocket money' (nie- to niema tego samego znaczenia - pocket money to sa te pieniadze ktore dajemy malym dzieciom, jak pracujemy to mowimy 'any spare cash') back to the Head Teacher. It will be difficult for you
but (I am sure that) you will feel 'yourself far better' (zla kol slow ...far better in yourself) when you will pay off 'one's' (nie uzywaj tego slowa - tu nie mowisz o jakiejs osobie ktorej nie znasz, tu zwrcasz sie do kolezanki-your). 'You must' (You must, wg mnie to wyglada jak rozkaz - lepiej - Please try) try.
I believe that you 'will' CAN cope with 'it' THIS and THAT you will pay for the broken computer. Don't give up, it will be 'well' (zle slowo - tutaj potrzeba All right)!
'Successes' (nie, tak nie mowimy - moze 'Best wishes', albo 'All the best')