Grouped moving average in r - r

Question

I'm trying to calculate a moving average in r over a particular field BUT I need this moving average to be grouped by two or more other fields. The purpose of this new average is for predictive analysis so I need it to be trailing as well.
Any variables that do not have enough values to be averaged (such as student J) would ideally give either NA or its original Score value.
I've been trying rollapply and data.table and am having no luck!
I've provided the table of data and two moving averages (AVG2 with k=2 and AVG3 with k=3) to show exactly what I'm after. The moving average is on Score and the variables to group over are school, Student and area. Please help!
no school Student area Score **AVG2** **AVG3**
1 I S A 5 NA NA
2 B S A 2 NA NA
3 B S A 7 NA NA
4 B O A 3 NA NA
5 B O B 9 NA NA
6 I O A 6 NA NA
7 I O B 3 NA NA
8 I S A 7 NA NA
9 I O A 1 NA NA
10 B S A 7 4.5 NA
11 I S A 3 NA NA
12 I O A 8 3.5 NA
13 B S A 3 7 5.33
14 I O A 4 4.5 5
15 B O A 1 NA NA
16 I S A 9 5 5
17 B S A 4 5 5.67
18 B O A 6 2 NA
19 I S A 3 6 6.33
20 I O B 8 NA NA
21 B S A 3 3.5 4.67
22 I O A 4 6 4.33
23 B O A 1 3.5 3.33
24 I S A 9 6 5
25 B S A 4 3.5 3.33
26 B O A 6 3.5 2.67
27 I J A 6 NA NA
here is the code to recreate the initial table in r:
school <- c('I','B','B','B','B','I','I','I','I','B','I','I','B','I','B','I','B','B','I','I','B','I','B','I','B','B','I')
Student <- c('S','S','S','O','O','O','O','S','O','S','S','O','S','O','O','S','S','O','S','O','S','O','O','S','S','O','J')
area <- c('A','A','A','A','B','A','B','A','A','A','A','A','A','A','A','A','A','A','A','B','A','A','A','A','A','A','A')
Score <- c(5,2,7,3,9,6,3,7,1,7,3,8,3,4,1,9,4,6,3,8,3,4,1,9,4,6,6)
data.frame(school, Student, area, Score)

Answer 1

You can try solving the problem using dplyr and TTR but for student J from school I it is not possible to calculate a moving average as there's only one measurement.
AVG2 caluculated with stats:filter gives the result you wanted to have, but I also added AVG2b calculated with TTR::SMA to show a simple moving average calculation, where the current measurement is also taken into account.
library(dplyr)
library(TTR)
df <- data.frame(school, Student, Score)
df$AVG2 <- NA
df$AVG2b <- NA
df[!(df$school=="I" & df$Student=="J"),] <- df[!(df$school=="I" & df$Student=="J"),] %>%
group_by(school, Student) %>%
mutate(AVG2 = stats::filter(Score, c(0, 0.5, 0.5), sides = 1 ), AVG2b = SMA(Score, n= 2))
> df
school Student Score AVG2 AVG2b
1 I S 5 NA NA
2 B S 2 NA NA
3 B S 7 NA 4.5
4 B O 3 NA NA
5 B O 9 NA 6.0
6 I O 6 NA NA
7 I O 3 NA 4.5
8 I S 7 NA 6.0
9 I O 1 4.5 2.0
10 B S 7 4.5 7.0
...

Answer 2

Here is a rollapply solution. Note that it appears that you want the average of the prior two or three rows in the same group, i.e. excluding the data on the current row.
library(zoo)
roll <- function(x, n) {
if (length(x) <= n) NA
else rollapply(x, list(-seq(n)), mean, fill = NA)
}
transform(DF, AVG2 = ave(Score, school, Student, FUN = function(x) roll(x, 2)),
AVG3 = ave(Score, school, Student, FUN = function(x) roll(x, 3)))
giving:
school Student Score AVG2 AVG3
1 I S 5 NA NA
2 B S 2 NA NA
3 B S 7 NA NA
4 B O 3 NA NA
5 B O 9 NA NA
6 I O 6 NA NA
7 I O 3 NA NA
8 I S 7 NA NA
9 I O 1 4.5 NA
10 B S 7 4.5 NA
11 I S 3 6.0 NA
12 I O 8 2.0 3.333333
13 B S 3 7.0 5.333333
14 I O 4 4.5 4.000000
15 B O 1 6.0 NA
16 I S 9 5.0 5.000000
17 B S 4 5.0 5.666667
18 B O 6 5.0 4.333333
19 I S 3 6.0 6.333333
20 I O 8 6.0 4.333333
21 B S 3 3.5 4.666667
22 I O 4 6.0 6.666667
23 B O 1 3.5 5.333333
24 I S 9 6.0 5.000000
25 B S 4 3.5 3.333333
26 B O 6 3.5 2.666667
27 I J 6 NA NA
Update: Fixed roll.

Answer 3

Here is AVG2 calculation with data.table, which is faster compared to other approaches:
library(data.table)
dt <- data.table(df)
setkey(dt, school, Student, area)
dt[, c("start", "len") := .(ifelse(.I + 1 > .I[.N], 0, .I +1), pmax(pmin(1, .I[.N] - .I -1), 0)), by = .(school, Student, area)][
, AVG2 := mean(dt$Score[start:(start+len)]), by = 1:nrow(dt)]
res$AVG2[res$len == 0] <- NA