Another thing to check on is that the bill matches the actual readings on the meter. Locally where I am there has been quite a fuss lately with people getting anomalous bills for gas. It turns out that the utility has been sending some bills with ‘estimates’ rather than actual meter readings.
Some local councils offer energy audits on occasions either free or or at low cost. A person will review energy accounts and look at appliances, insulation, patterns of energy use and suggest things to reduce consumption.
Could it be that your kids are old enough to be at home when you are not and staying in the shower for ages? Heating water is a large consumer of energy, especially if you have a teenager who likes to shower!! Kids can do things like run the heater with the window open too!
A faulty pressure relief valve on the water heater letting a lot of water out intermittently would be a conceivable expensive fault.