I think the show was slightly rushed. According to Gary Barlow, the reason there were so many songwriters on board was because there was so little time to write the songs. If the song writers were a small, select team, the songs would have flowed better and there wouldn't be so many good songs bogged down with a load of average ones which did let the show down slightly. I've read that the casting was quite late for some characters as well. Everything had to be done pretty quickly, when the project first started they had nothing, just a very basic concept and I think this showed in the programme. Some episodes were very hit and miss.
As people have mentioned, it wasn't a very good timeslot for the show. Although they did it with good intentions, having a show in a slot which is usually occupied by shows for older audiences, it was definitely a hinderance. Putting it between the X Factor would have gelled well with the music theme and would have attracted a huge audience just for being in between the X Factor. I don't see why the couldn't have moved their other shows to a Sunday like Family Fortunes as they already had an established audience who would have watched regardless of the time it was on.
The promotion of the show wasn't very good. They made it look like something aimed at a much younger audience than it was. It had storylines about multiple sclerosis and teacher/student relationships which is definitely more adult than anything in High School Musical.
The show had the potential to be very successful because is it just the type of show that is very 'now' but ITV messed up big time. It's a shame if Britannia High doesn't get recommissioned because I think these mistakes are easily corrected.