If you have some knowledge in php and Sql its not a big task.
"Not a big task" is relative. Even with something like a basis SIS, you have three separate applications with a common data store to create -- the student access portal, the teacher/instructor portal, and the educational administration portal, each of which has its own security and confidentiality guidelines to encode in the system, many of which would be mandated by statute. Now consider that individual test and assignment marks are not created equal -- some carry a heavier weight than others, some may be letter grades only, some are merely pass/fail, some are percentages and others are fractional (you got 17/18 on this quiz). Your interfaces need to account for all of these things in a way that makes it easy for the end-users (students, teaches and administrators) to use. Oh, and you'll need to be able to do trending and other statistics on the fly and as consolidated reports, often without the person looking for trends having access to all of the data under their user ID. Then there are the subjective student assessments/evaluations (and usually similar evals for teachers, TAs, and so forth) and the maintenance of draft (unpublished) versions of all of those.
Yes, you will get something that's an exact fit if you do custom development. But it's going to take a while to get there -- even requirements gathering is going to take a couple of weeks on the most optimistic schedule. And a developer who just throws a UI at a user group without consulting and testing (paper prototyping, etc.) ought to be locked in a basement somewhere to do nothing more potentially harmful than optimizing an implementation of A*. Then allow for the fact that version 1.0 is never satisfactory, even if it does fulfill the basic requirement -- you need to be able to devote people to iteration and improvement.
Coding is easy. Software engineering and development is hard. The developer doesn't get to make all of the choices.
So this "not a big task" is very likely to become 3-6 months of hard slogging under the best of circumstances if you care at all about doing it right. Yes, you'll have something that's the best possible fit at the end of it all, but it's not something you can knock off in a weekend unless you really expect your users to devote a whole lot of
their (more valuable than
your) time learning to use what you decided to give them. And you probably will have forgotten to include many of the most important parts simply because you didn't know they needed to be there.
Now compare that reality with an off-the-shelf solution. Someone else has already gone though all of those problems, and has fixed most of them. All you need to do is spend a little bit of time reading the doco, installing/deploying and configuring. It may not be a perfect fit, but it's a week or two rather than months to roll it out. Whether you are only considering the time as an absolute or are including the development costs, off-the-shelf is a whole lot cheaper even if there's a price tag on the shrinkwrap.
