Some amount of American school spending also goes towards addressing those problems, not really towards education, which can distort what the "education spending" figures mean.
For example, depending on which figures for education spending you use, it might include the free school lunch program, which is really more a kind of anti-poverty program that happens to be distributed via the schools. In addition, beefed up school security, anti-gang programs, and counseling programs are the school acting as policeman and social worker more than as educator. So it could be (but I can't say for sure) that Finland's lower per-student education budget still includes more actual spending on education, if other parts of the government take care of the other issues better.
Finland has free school lunches for everyone, and I guess counseling as well, but the other issues you mention aren't handled by schools as far as I know.
For example, depending on which figures for education spending you use, it might include the free school lunch program, which is really more a kind of anti-poverty program that happens to be distributed via the schools. In addition, beefed up school security, anti-gang programs, and counseling programs are the school acting as policeman and social worker more than as educator. So it could be (but I can't say for sure) that Finland's lower per-student education budget still includes more actual spending on education, if other parts of the government take care of the other issues better.