Primary and lower secondary education is compulsory in all European countries (with the exception of Vatican City), but there are huge differences in the actual number of hours children are required to spend at school. The following map shows the total number of hours of instruction in compulsory education; average total times spent in school would probably be closer to each other, as children in countries with shorter compulsory education are more likely to take additional (non-compulsory) classes.
The map is based on a study by OECD and is therefore limited only to OECD member and partner countries. It shows an interesting geographical trend—the number is very high in Western European countries and decreases towards the East (but I have no idea whether this is just a coincidence or a real pattern). Also note that some countries allow alternative schooling methods, such as homeschooling, which are not reflected in the map.