I don't think that is all that unusual through middle school and even the first year or so of high school. I always say I got through school by osmosis until about 11th grade when I started hitting some very advanced courses. The time will come when you will inevitably be faced with a course that will force you to start actually studying. Hopefully when that happens you will rise to the challenge.

If you grades are starting to suffer, it is because that challenge has begun and you haven't changed your habits to meet those requirements. Time to set aside time each night to take inventory of what you need to do as homework, what upcoming assignments/tests are on their way, etc. Osmosis doesn't work forever.